Communication device, noise removing method, and program

ABSTRACT

A communication device according to an embodiment of the present invention includes a communication antenna that receives a transmission signal where a spectrum spread signal subjected to a spectrum spread is modulated; an intermediate frequency converting unit that converts the transmission signal received by the communication antenna into an intermediate frequency signal having a predetermined frequency; an analog to digital converting unit that discretizes the intermediate frequency signal and outputs a discretization signal; a noise removing unit that detects a noise other than a normal thermal noise included in the discretization signal and removes the detected noise from the discretization signal; and a demodulating unit that demodulates the spectrum spread signal, based on the discretization signal that is output from the noise removing unit.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a division of application Ser. No. 12/392,596, filed Feb. 25,2009, now U.S. Pat. No. 8,243,776 which claims the priority benefits toJapanese Patent Application No. 2008-044496 filed in the Japan PatentOffice on Feb. 26, 2008, the entire contents of which are beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication device, a noiseremoving method, and a program.

2. Description of the Related Art

In recent years, a GPS (Global Positioning System) function is mountedin various electronic apparatuses, such as a car navigation device, aportable communication device like a mobile phone, or a digital stillcamera. The electronic apparatus includes a GPS receiving device (anexample of a communication device) that receives a transmission signalcalled a GPS L1 C/A code signal that is transmitted from four or moreGPS satellites and specifies a position based on the receivedtransmission signal or functions as a GPS receiving device, therebyrealizing the GPS function.

The GPS receiving device that is included in the electronic apparatusdemodulates a signal from each GPS satellite to acquire orbital data ofeach GPS satellite, derives a three-dimensional position of the GPSreceiving device by a simultaneous equation using the orbital data, timeinformation, and a delay time of the received signal, and specifies theposition. In this case, the GPS receiving device uses orbital data of aplurality of GPS satellites in order to remove an influence due to anerror between internal time information of the GPS receiving device andtime information in each GPS satellite.

In this case, a transmission signal that is transmitted from each GPSsatellite is a signal that is obtained by performing BPSK (Binary PhaseShift Keying) modulation on a carrier of 1575.42 MHz, based on aspectrum spread signal that is obtained by subjecting data of 50 bps toa spectrum spread using a Gold code where a code length is 1023 and achip rate is at 1.023 MHz. Accordingly, when the GPS receiving devicereceives a transmission signal from each GPS satellite and demodulates atransmitted spectrum spread signal, it may be requested to synchronize aspread code, a carrier, and data.

A technology that is related to a communication device that receives atransmission signal where a spectrum spread signal subjected to aspectrum spread is modulated has been developed. A technology thatquickly performs synchronous capturing of a spread code in atransmission signal transmitted from a GPS satellite is disclosed inPatent Document 1 as one example.

Meanwhile, the electronic apparatus that has a GPS function has hadmultifunction and high performance, and unnecessary radiation that isgenerated in the electronic apparatus has increased. The unnecessaryradiation that is generated in the electronic apparatus corresponds toan external noise due to a GPS receiving device (or GPS receiving deviceof an electronic apparatus, hereinafter, this is applicable) that isincluded in the electronic apparatus. In this case, representativeexamples that become factors of the external noise due to theunnecessary radiation may include a clock signal that interferes throughcoupling or space between wiring lines in the electronic apparatus,harmonics due to a high speed signal that passes through a data bus, avariation in load of a circuit, or a variation in power due to aswitching regulator.

When the external noise is applied to an analog circuit of the GPSreceiving device, if the external noise has the same level as a normalthermal noise (for example, −111 [dBm] when the thermal noise isconverted into a bandwidth of 2 MHz) that is generated in the GPSreceiving device, the GPS receiving device can normally demodulate thespectrum spread signal. However, when the external noise is strongerthan the normal thermal noise that is generated in the GPS receivingdevice, reception sensitivity is deteriorated by the amount exceedingthe level of the normal thermal noise. If the reciprocal of a “ratiobetween a received transmission signal and a thermal noise+an externalnoise” (S/(N+I)) approximates to a process gain, the GPS receivingdevice may not normally demodulate the spectrum spread signal.

Accordingly, for example, as in the GPS receiving device, in acommunication device that receives a transmission signal where aspectrum spread signal subjected to a spectrum spread is modulated, aninfluence due to the external noise needs to be removed as much aspossible, in order to normally demodulate the spectrum spread signal.

Among them, a technology for removing an external noise in acommunication device that receives a transmission signal where aspectrum spread signal subjected to a spectrum spread is modulated hasbeen developed. In order to remove the external noise in the GPSreceiving device, a technology using a notch filter (it may also bereferred to as a band-elimination filter) is disclosed in a Non-PatentDocument 1 as one example.

[Patent Document 1] JP-A-2003-232844

[Non-Patent Document 1] Daniele Borio, Laura Camoriano, Paolo Mulassano,“Analysis of the One-Pole Notch Filter for Interference Mitigation:Wiener Solution and Loss Estimation”, ION GNSS 19th InternationalTechnical Meeting of the Satellite Division, 26-29 Sep. 2006, pp.1849-1860.

The communication device where the technology for removing an externalnoise according to the related art is applied includes a notch filterthat attenuates a signal corresponding to a set notch frequency. Thecommunication device controls the notch frequency of the notch filter,thereby removing the external noise that is generated due to theunnecessary radiation. In this case, the communication device where thetechnology for removing an external noise according to the related artis applied uses an LMS (Least Mean Square) algorithm that is generallyused in an adaptive filter, thereby controlling the notch frequency ofthe notch filter. However, in the LMS algorithm where the technology forremoving an external noise according to the related art is used, inorder to control the notch frequency, a feedback operation needs to beperformed on a signal that is used to remove the external noise. Forthis reason, in the communication device where the technology forremoving an external noise according to the related art is applied, itis necessary to consider convergence as a control loop. That is, in thecommunication device where the technology for removing an external noiseaccording to the related art is applied, an unstable operation, such asdivergence, may be generated depending on setting of the control loop ora signal (signal used to remove the external noise) input to the notchfilter.

Accordingly, the communication device using the technology for removingan external noise according to the related art cannot stably remove theexternal noise. Further, no description is given to a method of removingthe external noise in the communication device according to the relatedart that quickly performs synchronous capturing of a spread code. Eventhough the technologies according to the related art are combined, itmay not be possible to stably remove the external noise.

Further, as a countermeasure that removes an influence due to theexternal noise as much as possible, for example, a shield material or ashield case is used in the communication device (or electronic apparatuswhere the communication device is mounted), thereby suppressingunnecessary radiation. However, since the above countermeasure affects adesign or cost of the communication device (or electronic apparatuswhere the communication device is mounted), the above countermeasure isnot preferable.

SUMMARY OF THE INVENTION

Accordingly, the present invention addresses the above-identified andother issues associated with conventional methods and apparatuses. Thereis a need for a communication device, a noise removing method, and aprogram that can stably remove an external noise from a transmissionsignal where a spectrum spread signal is modulated.

According to an embodiment of the present invention, there is provided acommunication device. The communication device includes a communicationantenna that receives a transmission signal where a spectrum spreadsignal subjected to a spectrum spread is modulated; an intermediatefrequency converting unit that converts the transmission signal receivedby the communication antenna into an intermediate frequency signalhaving a predetermined frequency; an analog to digital converting unitthat discretizes the intermediate frequency signal and outputs adiscretization signal; a noise removing unit that detects a noise otherthan a normal thermal noise included in the discretization signal andremoves the detected noise from the discretization signal; and ademodulating unit that demodulates the spectrum spread signal, based onthe discretization signal that is output from the noise removing unit.

By this configuration, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

The noise removing unit may include a first Fourier transforming unitthat performs a fast Fourier transform on the discretization signal; afrequency detecting unit that detects a frequency whose amplitude is apredetermined value or more as a peak frequency of the noise, based on aresult of the fast Fourier transform in the first Fourier transformingunit; and a notch filter that sets the peak frequency detected in thefrequency detecting unit as a notch frequency, and outputs adiscretization signal where a frequency component corresponding to theset notch frequency is attenuated based on the discretization signal.

By this configuration, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

The noise removing unit may further include a first adaptive filter thatoutputs a discretization signal that minimizes a mean squared error withrespect to an ideal discretization signal in an ideal state notincluding the noise.

By this configuration, it is possible to stably remove a singlefrequency noise, a narrowband noise where a noise band is narrow, and abroadband noise.

The communication device may further include a level detecting unit thatdetects an average value or an integration value in a predetermined timeof the discretization signal; and an adjustment signal output unit thatoutputs an adjustment signal to selectively operate the notch filter andthe first adaptive filter, based on the average value or the integrationvalue detected by the level detecting unit and a predetermined thresholdvalue.

By this configuration, an S/N ratio of a spectrum spread signal can beprevented from being lost due to removing of a noise.

The noise removing unit may further include a plurality of notchfilters, and in the notch filters, the peak frequencies that aredetected by the frequency detecting unit may be set in the order of thepeak frequencies having large amplitude.

By this configuration, it is possible to stably remove a plurality ofsingle frequency noises or narrowband noises where a noise band isnarrow.

The frequency detecting unit may periodically or randomly perform a fastFourier transform on the discretization signal, and the notch filter mayset the notch frequency, when the peak frequency is detected in thefrequency detecting unit.

By this configuration, it is possible to remove an external noise whilereducing a loss of consumed power.

The noise removing unit may include a second Fourier transforming unitthat performs a fast Fourier transform on the discretization signal andderives a power spectrum based on a result of the fast Fouriertransform; and a first Wiener filter that outputs a discretizationsignal that minimizes a mean squared error with respect to an idealdiscretization signal in an ideal state not including the noise, basedon the discretization signal, the power spectrum output from the secondFourier transforming unit, and reference power per unit frequency.

By this configuration, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

The noise removing unit may include a third Fourier transforming unitthat performs a fast Fourier transform on the discretization signal; afirst determining unit that selectively outputs a result of the fastFourier transform in the third Fourier transforming unit or a powerspectrum derived from the result of the fast Fourier transform, based onthe result of the fast Fourier transform and the power spectrum; asecond Wiener filter that outputs a discretization signal that minimizesa mean squared error with respect to an ideal discretization signal inan ideal state not including the noise, based on the power spectrum andreference power per unit frequency, when the power spectrum is outputfrom the first determining unit; and an inversed Fourier transformingunit that performs an inversed fast Fourier transform on the result ofthe fast Fourier transform output from the first determining unit or thediscretization signal output from the second Wiener filter.

By this configuration, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

The noise removing unit may include a plurality of band-pass filterseach of which detects a predetermined frequency band detection signalfrom the discretization signal; a plurality of second adaptive filtersthat correspond to the band-pass filters, and selectively output thedetection signal or a discretization signal that minimizes a meansquared error with respect to an ideal discretization signal in an idealstate not including the noise; and a synthesizing unit that synthesizesthe discretization signals output from the second adaptive filters. Eachof the second adaptive filters may include a second determining unitthat selectively outputs the detection signal or a power spectrumderived from the detection signal, based on the power spectrum; and athird Wiener filter that outputs a discretization signal that minimizesa mean squared error with respect to the ideal discretization signal,based on the power spectrum and reference power per unit frequency, whenthe power spectrum is output from the second determining unit.

By this configuration, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

The noise removing unit may detect the noise other than the normalthermal noise included in the discretization signal without performing afeedback operation on the discretization signal.

By this configuration, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

The analog to digital converting unit may include an analog to digitalconverter that has a resolution of N bits (N is an integer) larger thanthe number of bits corresponding to average amplitude of the normalthermal noise, and converts an input analog signal into a digitalsignal, and the analog to digital converter sets the average amplitudeof the normal thermal noise as lower M bits (M is an integer whereN>·M).

By this configuration, an external noise can be more surely removed froma transmission signal where a spectrum spread signal is modulated.

The communication device may further include a bit number determiningunit that sets the number of bits of the discretization signal outputfrom the noise removing unit as P bits (P is an integer where N>·P).

By this configuration, it is possible to reduce the sizes of an operatoror a register and a memory that constitute a demodulating unit that candemodulate a spectrum spread signal.

The communication device may further include a frequency converting unitthat converts a signal into a discretization signal where a centralfrequency of the discretization signal output from the analog to digitalconverting unit is set as zero, the intermediate frequency convertingunit sets the predetermined frequency as a frequency other than zero,and the noise removing unit receives the discretization signal that isoutput from the frequency converting unit.

By this configuration, it is possible to improve easiness of a processof removing a noise.

According to the embodiments of the present invention described above,there is provided a noise removing method that can use a communicationdevice including a communication antenna that receives a transmissionsignal where a spectrum spread signal subjected to a spectrum spread ismodulated, an intermediate frequency converting unit that converts thetransmission signal received by the communication antenna into anintermediate frequency signal having a predetermined frequency, and ananalog to digital converting unit that discretizes the intermediatefrequency signal and outputs a discretization signal, and removes anoise other than a normal thermal noise included in the discretizationsignal. The noise removing method includes the steps of: performing afast Fourier transform on the discretization signal; detecting afrequency whose amplitude is a predetermined value or more as a peakfrequency of the noise, based on a result of the fast Fourier transformin the performing of the fast Fourier transform; and outputting adiscretization signal where a frequency component corresponding to thepeak frequency set detected in the detecting of the frequency isattenuated, based on the discretization signal.

By using this method, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

According to the embodiments of the present invention described above,there is provided a noise removing method that can use a communicationdevice including a communication antenna that receives a transmissionsignal where a spectrum spread signal subjected to a spectrum spread ismodulated, an intermediate frequency converting unit that converts thetransmission signal received by the communication antenna into anintermediate frequency signal having a predetermined frequency, and ananalog to digital converting unit that discretizes the intermediatefrequency signal and outputs a discretization signal, and removes anoise other than a normal thermal noise included in the discretizationsignal. The noise removing method includes the steps of: performing afast Fourier transform on the discretization signal and deriving a powerspectrum based on a result of the fast Fourier transform; and outputtinga discretization signal that minimizes a mean squared error with respectto an ideal discretization signal in an ideal state not including thenoise, based on the discretization signal, the power spectrum derived inthe deriving of the power spectrum, and reference power per unitfrequency.

By using this method, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

According to the embodiments of the present invention described above,there is provided a program that can be used in a communication deviceincluding a communication antenna that receives a transmission signalwhere a spectrum spread signal subjected to a spectrum spread ismodulated, an intermediate frequency converting unit that converts thetransmission signal received by the communication antenna into anintermediate frequency signal having a predetermined frequency, and ananalog to digital converting unit that discretizes the intermediatefrequency signal and outputs a discretization signal, and removes anoise other than a normal thermal noise included in the discretizationsignal. The program allows a computer to execute the steps of:performing a fast Fourier transform on the discretization signal;detecting a frequency whose amplitude is a predetermined value or moreas a peak frequency of the noise, based on a result of the fast Fouriertransform in the performing of the fast Fourier transform; andoutputting a discretization signal where a frequency componentcorresponding to the peak frequency set detected in the detecting of thefrequency is attenuated, based on the discretization signal.

By using this program, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

According to the embodiments of the present invention described above,there is provided a program that can be used in a communication deviceincluding a communication antenna that receives a transmission signalwhere a spectrum spread signal subjected to a spectrum spread ismodulated, an intermediate frequency converting unit that converts thetransmission signal received by the communication antenna into anintermediate frequency signal having a predetermined frequency, and ananalog to digital converting unit that discretizes the intermediatefrequency signal and outputs a discretization signal, and removes anoise other than a normal thermal noise included in the discretizationsignal. The program allows a computer to execute the steps of:performing a fast Fourier transform on the discretization signal andderiving a power spectrum based on a result of the fast Fouriertransform; and outputting a discretization signal that minimizes a meansquared error with respect to an ideal discretization signal in an idealstate not including the noise, based on the discretization signal, thepower spectrum derived in the deriving of the power spectrum, andreference power per unit frequency.

By using this program, an external noise can be stably removed from atransmission signal where a spectrum spread signal is modulated.

According to the embodiments of the present invention described above,an external noise can be stably removed from a transmission signal wherea spectrum spread signal is modulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a portion of the configuration of acommunication device according to the related art.

FIG. 2 is a diagram illustrating an example of a discretization signalthat includes an external noise.

FIG. 3A is a diagram illustrating an example of an output spectrum of anA/D converter in the case where an output of an A/D converter issaturated.

FIG. 3B is a diagram illustrating an example of an output spectrum of anA/D converter in the case where an output of an A/D converter issaturated.

FIG. 4A is a first diagram illustrating a result of the case where anoutput of an A/D converter is saturated in a communication deviceaccording to the related art.

FIG. 4B is a first diagram illustrating a result of the case where anoutput of an A/D converter is saturated in a communication deviceaccording to the related art.

FIG. 5A is a second diagram illustrating a result of the case where anoutput of an A/D converter is saturated in a communication deviceaccording to the related art.

FIG. 5B is a second diagram illustrating a result of the case where anoutput of an A/D converter is saturated in a communication deviceaccording to the related art.

FIG. 6 is a diagram illustrating an example of the configuration of acommunication device according to a first embodiment of the presentinvention.

FIG. 7 is a diagram illustrating an example of the configuration of anoise removing unit according to a first embodiment of the presentinvention.

FIG. 8 is a diagram illustrating an example of the configuration of anotch filter that is included in a communication device according to afirst embodiment of the present invention.

FIG. 9A is a diagram illustrating an effect of when a communicationdevice according to a first embodiment of the present invention includesa noise removing unit.

FIG. 9B is a diagram illustrating an effect of when a communicationdevice according to a first embodiment of the present invention includesa noise removing unit.

FIG. 10 is a diagram illustrating the configuration of a noise removingunit according to a modification of a first embodiment of the presentinvention.

FIG. 11 is a flowchart illustrating a noise removing method according toa first embodiment of the present invention.

FIG. 12 is a diagram illustrating another example of a discretizationsignal that includes an external noise.

FIG. 13 is a diagram illustrating an example of the configuration of anoise removing unit according to a second embodiment of the presentinvention.

FIG. 14 is a flowchart illustrating a noise removing method according toa second embodiment of the present invention.

FIG. 15 is a diagram illustrating an example of the configuration of anoise removing unit according to a third embodiment of the presentinvention.

FIG. 16A is a diagram illustrating an example of a fast Fouriertransform process and an inversed fast Fourier transform process in anoise removing unit according to a third embodiment of the presentinvention.

FIG. 16B is a diagram illustrating an example of a fast Fouriertransform process and an inversed fast Fourier transform process in anoise removing unit according to a third embodiment of the presentinvention.

FIG. 17 is a flowchart illustrating a noise removing method according toa third embodiment of the present invention.

FIG. 18 is a diagram illustrating an example of the configuration of anoise removing unit according to a fourth embodiment of the presentinvention.

FIG. 19 is a diagram illustrating an example of an output characteristicof a frequency sampling filter that is included in a noise removing unitaccording to a fourth embodiment of the present invention;

FIG. 20 is a diagram illustrating an example of the configuration of anadaptive filter that is included in a noise removing unit according to afourth embodiment of the present invention.

FIG. 21A is a diagram illustrating an effect of when a communicationdevice according to a fourth embodiment of the present inventionincludes a noise removing unit.

FIG. 21B is a diagram illustrating an effect of when a communicationdevice according to a fourth embodiment of the present inventionincludes a noise removing unit.

FIG. 22 is a diagram illustrating an example of the configuration of anoise removing unit according to a fifth embodiment of the presentinvention.

FIG. 23 is a diagram illustrating a portion of an example of theconfiguration of a communication device according to a modification of afifth embodiment of the present invention.

FIG. 24 is a diagram illustrating a portion of an example of theconfiguration of a communication device according to a sixth embodimentof the present invention.

FIG. 25 is a diagram illustrating a portion of an example of theconfiguration of a communication device according to a seventhembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note thatin this specification and the appended drawings, structural elementsthat have substantially the same functions and structures are denotedwith the same reference numerals and a repeated explanation of thesestructural elements is omitted.

(Issues in a Communication Device According to the Related Art)

Before describing a communication device according to an embodiment ofthe present invention, first, issues in the communication deviceaccording to the related art will be described.

[Configuration of a Communication Device According to the Related Art]

FIG. 1 is a diagram illustrating a portion of the configuration of acommunication device 10 according to the related art. In FIG. 1, ademodulating unit that demodulates a spectrum spread signal is notshown.

Referring to FIG. 1, the communication device 10 includes acommunication antenna 12, a frequency converting unit 14, and a notchfilter 16. In addition, an output x(t) of the notch filter 16 is inputto the demodulating unit (not shown).

Further, the communication device 10 may further include, for example, acontrol unit (not shown) that is configured using an MPU (MicroProcessing Unit) and can control the entire communication device 10, aROM (Read Only Memory) (not shown) where recorded is control data, suchas a program or an operation parameter, which is used by the controlunit, a RAM (Random Access Memory) (not shown) that primarily stores aprogram executed by the control unit, and a storage unit (not shown)that can store a variety of data, such as display data for an userinterface, or an application. The communication device 10 connects theabove-described constituent elements using a bus that functions as adata transmission path, for example. Further, the control unit (notshown) can function as the demodulating unit (not shown).

In this case, as the storage unit (not shown), for example, a magneticrecording medium, such as a hard disk, or a nonvolatile memory, such asa flash memory, is exemplified, but the present invention is not limitedthereto.

The communication antenna 12 receives a transmission signal that istransmitted from an external device, such as a GPS satellite. In thiscase, the transmission signal is a signal that is obtained by modulatinga spectrum spread signal subjected to spectrum spread. Examples of thetransmission signal may include a signal that is obtained by performingBPSK modulation on a carrier of 1575.42 MHz, based on a spectrum spreadsignal that is obtained by subjecting data of 50 bps to spectrum spreadusing a Gold code where a code length is 1023 and a chip rate is at1.023 MHz. In the description below, the transmission signal that istransmitted from the external device is also referred to as an RF (RadioFrequency) signal.

The frequency converting unit 14 includes a low noise amplifier 20(hereinafter, also referred to as “LNA”), an intermediate frequencyconverting circuit 22, an amplifier 24, a band-pass filter 26(hereinafter, also referred to as “BPF”), and an A/D converter 28(Analog to Digital Converter).

The LNA 20 amplifies an RF signal that is received by the communicationantenna 12. The intermediate frequency converting circuit 22 converts(down-converts) a frequency of the RF signal, which is amplified by theLNA 20, into an intermediate frequency (hereinafter, also referred to as“IF”) that is lower than a carrier frequency, such as, for example,4.092 MHz or 1.023 MHz, such that digital signal processing can beeasily executed.

The amplifier 24 is composed, for example, of an operational amplifier,and amplifies an IF signal whose frequency has been converted from thecarrier frequency that is output from the intermediate frequencyconverting circuit 22. The BPF 26 passes only a specific frequency bandsignal with respect to the amplified IF signal that is output from theamplifier 24, and attenuates the other band signals. In this case, thesignal that is processed by the elements from the LNA 20 to the BPF 26is an analog signal.

The A/D converter 28 discretizes a signal based on an analog IF signalAy(t) that is output from the BPF 26, and outputs an IF signal y(t) as adigital signal. Hereinafter, the IF signal as the digital signal isreferred to as a discretization signal.

By the above-described configuration, the frequency converting unit 14can convert the frequency of the RF signal received by the communicationantenna 12 into the IF signal that is down-converted to the intermediatefrequency (IF), and output the discretization signal y(t).

FIG. 2 is a diagram illustrating an example of a discretization signaly(t) that includes an external noise, which shows an output spectrum(center of 4 MHz) of an A/D converter. In this case, the discretizationsignal y(t) includes a spectrum spread signal that becomes ademodulation subject transmitted from the external device, a normalthermal noise that is generated by the communication device 10, and anexternal noise. Specifically, FIG. 2 shows an example of the case anarrowband noise using 4.1 MHz as a peak is included as an externalnoise. Further, as described above, in the spectrum spread signal thatis transmitted from the external device, such as the GPS satellite, itssignal level is generally lower than that of the external noise. In theexample of FIG. 2, a spectrum spread signal is buried in a normalthermal noise.

The notch filter 16 removes the external noise from the discretizationsignal y(t) that is output from the frequency converting unit 14, byapplying rapid attenuation to a specific frequency (notch frequency),and outputs a discretization signal x(t) where an external noisecorresponding to the notch frequency is removed.

Further, the notch filter 16 feeds backs the discretization signal x(t)where the external noise is removed with respect to the inputdiscretization signal y(t), thereby appropriately setting a notchfrequency. That is, the notch filter 16 includes a control loop thatfeed backs the discretization signal x(t) with respect to thediscretization signal y(t).

The discretization signal x(t) that is output from the notch filter 16is input to the demodulating unit (not shown). The demodulating unit(not shown) synchronizes a spread code, a carrier, and data to execute ade-spread process, thereby demodulating the spectrum spread signal.

Since the communication device 10 according to the related art includesthe above-described notch filter 16, the communication device 10 canremove a signal that corresponds to the appropriately set notchfrequency. That is, the communication device 10 controls the notchfrequency, thereby removing the external noise that is shown in FIG. 2.Further, the communication device 10 can demodulate the spectrum spreadsignal, when the external noise shown in FIG. 2 is removed.

[Issues of a Communication Device According to the Related Art]

[1] First Issue

Since the communication device 10 according to the related art includesthe notch filter 16, the communication device 10 can remove the signalthat corresponds to the notch frequency. However, the communicationdevice 10 is configured to set the notch frequency using the controlloop that feeds back the discretization signal x(t) where the externalnoise is removed with respect to the input discretization signal y(t).That is, in order to set the notch frequency that is effective when theexternal noise is removed, the communication device 10 needs to considerconvergence of the control loop. Accordingly, it is not possible to setthe notch frequency that is effective when the external noise isremoved, for example, using the setting of the control loop or thediscretization signal y(t) that is input to the notch filter 16.Alternatively, an unstable operation such as divergence of the controlloop, may be generated.

As described above, when the notch filter 16 performs an unstableoperation, the notch filter 16 cannot remove the external noise.Further, demodulation of the spectrum spread signal will be, of course,far from normalcy. Accordingly, the communication device 10 according tothe related art cannot stably remove the external noise.

[2] Second Issue

In the case where the external noise is applied to an analog circuit ofthe communication device, when the external noise is stronger than anormal thermal noise (for example, −111 [dBm] when the thermal noise isconverted into a band width of 2 MHz) that is generated in thecommunication device, if a reciprocal of a “ratio between a receivedtransmission signal (RF signal) and a thermal noise+an external noise”(hereinafter, referred to as “S/(N+I)” where “S” indicates the receivedtransmission signal, “N” indicates the thermal noise, and “I” indicatesthe external noise) approximates to process gain, the communicationdevice may not normally demodulate the spectrum spread signal. For thisreason, the communication device 10 according to the related artincludes the notch filter 16 to remove the external noise.

However, even when the reciprocal of S/(N+I) is sufficiently smallerthan the process gain, if the analog circuit is saturated due to thestrong external noise, the thermal noise and the spectrum spread signalto be demodulated are suppressed. As a result, it may not becomepossible to demodulate the spectrum spread signal. In this case, an A/Dconverter that outputs a discretization signal in a GPS receiving devicethat is an example of the communication device 10 according to therelated art has a resolution of, for example, 1 bit or 2 bits, because areception S/N of a GPS signal (an example of the spectrum spread signal)is significantly lower than 0 dB. In addition, in the GPS receivingdevice, the thermal noise and the spectrum spread signal are basicallyused in a state where they are saturated to some extent. Accordingly,when the external noise is applied, an output of the A/D converter thatis provided at a final stage of the analog circuit in the communicationdevice (for example, GPS receiving device) is particularly easy to besaturated.

FIGS. 3A and 3B are diagrams illustrating an example of an outputspectrum of an A/D converter in the case where an output of the A/Dconverter is saturated. Specifically, FIG. 3A shows the case where anoutput of an A/D converter is not saturated, and FIG. 3B shows the casewhere an output of an A/D converter is saturated. As shown in FIG. 3B,when the output of the A/D converter is saturated, information of the RFsignal that becomes an origin of the discretization signal may be lost.

FIGS. 4A and 4B are first diagrams illustrating a result of the casewhere an output of an A/D converter is saturated in a communicationdevice 10 according to the related art. Specifically, FIG. 4A shows anexample of an IF signal that is output from a BPF 26 of a communicationdevice 10. FIG. 4B shows a result of a de-spread process in ademodulating unit (not shown) in the case where an IF signal shown inFIG. 4A is not saturated in an A/D converter 28.

As shown in FIG. 4B, when an output of an A/D converter 28 is notsaturated, it can be recognized that a spectrum spread signal isdetected in a demodulating unit (not shown).

FIGS. 5A and 5B are second diagrams illustrating a result in the casewhere an output of an A/D converter is saturated in a communicationdevice 10 according to the related art. In this case, FIG. 5A shows anoutput spectrum of an A/D converter 28 in the case where an IF signalshown in FIG. 4A is saturated in an A/D converter 28. FIG. 5B shows aresult of a de-spread process in the case where an output spectrum shownin FIG. 5A is demodulated by a demodulating unit (not shown).

As shown in FIG. 5B, when an output of the A/D converter 28 issaturated, it can be recognized that a spectrum spread signal is notdetected in the demodulating unit (not shown).

As shown in FIGS. 4A, 4B, 5A, and 5B, when the output of the A/Dconverter 28 is saturated, the communication device 10 cannot detect thespectrum spread signal that is included in the IF signal output from theBPF 26, using the demodulating unit (not shown).

In this case, in the communication device 10, since no countermeasure istaken with respect to saturation of the output of the A/D converter 28that is provided at the final stage of the analog circuit, it is likelyfor the output of the A/D converter 28 to be saturated. In addition,when the output of the A/D converter 28 is saturated, in thecommunication device 10, the external noise cannot be removed, and thespectrum spread signal cannot be demodulated.

(Issue Resolving Approach in a Communication Device According to anEmbodiment of the Present Invention)

As described above, since the communication device 10 according to therelated art has the two issues, that is, the first issue and the secondissue, the spectrum spread signal could not be demodulated, even whenthe communication device 10 receives the spectrum spread signal that istransmitted from the external device. Accordingly, the communicationdevice according to the embodiment of the present invention takesapproaches described in the following (1) and (2) with respect to thefirst issue and the second issue.

(1) Approach with Respect to a First Issue

The communication device according to the embodiment of the presentinvention includes a noise removing unit that is provided at a rearstage of the A/D converter provided at the final stage of the analogcircuit. In addition, the communication device according to theembodiment of the present invention uses the noise removing unit todetect a noise other than a normal thermal noise included in adiscretization signal, that is, an external noise included in thediscretization signal without performing a feedback operation withrespect to the corresponding discretization signal, based on thereceived RF signal (transmission signal). In addition, the communicationdevice according to the embodiment of the present invention removes thedetected noise. In this case, the removing of the noise in thecommunication device according to the embodiment of the presentinvention is not limited to the removing of the external noise, butfurther includes reducing of the external noise.

Since the external noise can be detected without performing the feedbackoperation with respect to the discretization signal and the detectedexternal noise can be removed, the noise removing unit that removes thenoise does not cause an unstable operation depending on the setting ofthe control loop or the discretization signal, as in the communicationdevice 10 according to the related art. Accordingly, the communicationdevice according to the embodiment of the present invention can stablyremove the external noise from a transmission signal where the spectrumspread signal is modulated. Further, since the communication deviceaccording to the embodiment of the present invention can stably removethe external noise, the communication device can demodulate the spectrumspread signal more surely than the communication device 10 according tothe related art. The specific configuration of the communication deviceaccording to the embodiment of the present invention will be describedin detail below.

(2) Approach with Respect to a Second Issue

The communication device according to the embodiment of the presentinvention includes, as the A/D converter provided at the final stage ofthe analog circuit, an A/D converter that has a resolution of N bits (Nis an integer) that are larger than the number of bits corresponding toaverage amplitude of a normal thermal noise. Further, the communicationdevice according to the embodiment of the present invention sets theaverage amplitude of the normal thermal noise as lower M bits (M is aninteger that satisfies the condition N>M) of the A/D converter.

By the above-described configuration, the A/D converter according to theembodiment of the present invention is not saturated to the externalnoise stronger than the thermal noise by (N−M)×6 [dB]. In this case,when the GPS signal is received as an example of the spectrum spreadsignal, a reception S/N of the GPS signal becomes significantly lowerthan 0 [dB]. Therefore, the A/D converter according to the embodiment ofthe present invention can set M as M=1 or M=2, for example. At thistime, for example, if the A/D converter according to the embodiment ofthe present invention is composed of a 6-bit A/D converter (that is,N=6), even though an external noise whose level is 24 [dB] higher thanthat of the normal thermal noise is input, an output spectrum of the A/Dconverter is not saturated.

Further, it should be noted that the N value and the M value accordingto the embodiment of the present invention are not limited to N=6 andM=1 or M=2. For example, N that indicates a resolution of the A/Dconverter according to the embodiment of the present invention can bedetermined depending on the intensity of an input signal with which anoutput of the entire analog circuit provided at a previous stage of theA/D converter is saturated. Specifically, for example, when the analogcircuit at the previous stage of the A/D converter that is included inthe communication device according to the embodiment of the presentinvention is saturated with input intensity of −90 [dBm], it is possibleto spare about 21 [dB] as compared with the normal thermal noise.Therefore, N has a sufficiency of 6 bits. Accordingly, in the abovecase, the communication device according to the embodiment of thepresent invention can configure the A/D converter using an A/D converterthat has a resolution of 6 bits.

The communication device according to the embodiment of the presentinvention has the configuration where the output spectrum of the A/Dconverter can be prevented from being saturated due to the externalnoise, thereby surely removing the external noise in the noise removingunit that is provided at the rear stage of the A/D converter.Accordingly, the communication device according to the embodiment of thepresent invention can demodulate the spectrum spread signal more surelythan the communication device 10 according to the related art.

Next, the configuration of the communication device according to theembodiment of the present invention will be specifically described. Inthe description below, it is assumed that the communication deviceaccording to the embodiment of the present invention receives atransmission signal (hereinafter, also referred to as an “RF signal”)that is transmitted from the external device. In this case, thetransmission signal according to the embodiment of the present inventionis a signal that is obtained by modulating a spectrum spread signalsubjected to spectrum spread. Examples of the transmission signal mayinclude a signal that is obtained by performing BPSK modulation on acarrier of 1575.42 MHz, based on a spectrum spread signal that isobtained by subjecting data of 50 bps to spectrum spread using a Goldcode where a code length is 1023 and a chip rate is at 1.023 MHz, butthe present invention is not limited thereto. For example, thecommunication device according to the embodiment of the presentinvention can communicate with the external device using a CDMA (CodeDivision Multiple Access) scheme or a scheme based on the IEEE802.11standard.

(First Embodiment)

First, as a communication device according to a first embodiment,described is a communication device that includes a noise removing unitthat mainly removes a single frequency noise or a narrowband noise(external noise) where a band of a noise is narrow. In this case,examples of the narrowband noise may include a noise that is generatedby harmonics of a clock signal that drives a digital circuit, othernarrowband radio interference, and a power supply noise due to aswitching regulator, but the present invention is not limited thereto.

FIG. 6 is a diagram illustrating an example of the configuration of acommunication device 100 according to a first embodiment of the presentinvention. Referring to FIG. 6, the communication device 100 includes acommunication antenna 102, a frequency converting unit 104, a noiseremoving unit 106, a demodulating unit 108, an XO (X'tal Oscillator)110, and a TCXO (Temperature Compensated X'tal Oscillator) 112.

Further, the communication device 100 may include, for example, acontrol unit (not shown) that is composed of an MPU or the like and cancontrol the entire communication device 100, a ROM (Read Only Memory)(not shown) where recorded is control data, such as a program or anoperation parameter used by the control unit, a RAM (Random AccessMemory) (not shown) that primarily stores a program executed by thecontrol unit, a storage unit (not shown) that can store a variety ofdata, such as display data for a user interface, or an application, anoperation unit (not shown) that can be operated by a user, and a displayunit (not shown). The communication device 100 connects theabove-described various constituent elements, for example, using a busthat functions as a data transmission path. Further, the control unit(not shown) can function as the noise removing unit 106 or thedemodulating unit 108.

In this case, examples of the storage unit (not shown) may include, forexample, a magnetic recording medium, such as a hard disk, and anonvolatile memory, such as an EEPROM (Electrically Erasable andProgrammable Read Only Memory), a flash memory, an MRAM(Magnetoresistive Random Access Memory), an FeRAM (Ferroelectric RandomAccess Memory), and a PRAM (Phase change Random Access Memory), but thepresent invention is not limited thereto.

Further, examples of the operation unit (not shown) may include, forexample, an operation input device, such as a keyboard or a mouse, arotated selector, such as a button, a directional key, and a jog dial,or a combination thereof, but the present invention is not limitedthereto. Further, examples of the display unit (not shown) may include,for example, an LCD (Liquid Crystal Display), and an organic EL display(Organic ElectroLuminescence display; it may also be referred to as anOLED display (Organic Light Emitting Diode display)), but the presentinvention is not limited thereto. Further, the operation unit (notshown) and the display unit (not shown) can be integrally formed, like atouch screen.

The communication antenna 102 receives a transmission signal (RF signal)that is transmitted from an external device, such as a GPS satellite.

The frequency converting unit 104 converts a frequency of the RF signalthat is received by the communication antenna 102 into an IF signal(intermediate frequency signal) that is down-converted into anintermediate frequency (IF). In addition, the frequency converting unit104 discretizes a signal based on an analog IF signal, and outputs adiscretization signal. Hereinafter, an example of the configuration ofthe frequency converting unit 104 will be described.

[Example of the Configuration of a Frequency Converting Unit 104]

The frequency converting unit 104 includes a low noise amplifier 120(hereinafter, also referred to as an “LNA”), an intermediate frequencyconverting unit 122, an amplifier 124, a band-pass filter 126(hereinafter, the band-pass filter may also be referred to as a “BPF”),and an A/D converter 128.

The LNA 120 amplifies an RF signal that is received by the communicationantenna 102.

The intermediate frequency converting unit 122 converts (down-converts)the frequency of the RF signal amplified by the LNA 120 into anintermediate frequency (hereinafter, also referred to as an “IF”) lowerthan a carrier frequency of, for example, 4.092 MHz or 1.023 MHz, suchthat digital signal processing can be easily performed. In this case, anexample of the configuration of the intermediate frequency convertingunit 122 will be described.

[Example of the Configuration of an Intermediate Frequency ConvertingUnit 122]

The intermediate frequency converting unit 122 includes a band-passfilter 130, an amplifier 132, a frequency synthesizer 134, and a mixer136.

The BPF 130 passes only a specific frequency band signal with respect tothe amplified RF signal that is output from the LNA 120 and attenuatesthe other band signals.

The amplifier 132 amplifies an RF signal that is output from the BPF130. In this case, the amplifier 132 can be composed of, for example, aMOSFET (Metal Oxide Semiconductor Field effect transistor) differentialamplifier, but the present invention is not limited thereto.

The frequency synthesizer 134 generates a local oscillation signalhaving a predetermined frequency, based on an oscillation signal that issupplied from the TCXO 112 (which will be described in detail below). Inthis case, the frequency synthesizer 134 is controlled by, for example,an MPU 144 that is included in the demodulating unit 108, but thepresent invention is not limited thereto. The frequency synthesizer 134may be controlled by a control unit (not shown).

The mixer 136 multiplies the amplified RF signal output from theamplifier 132 by the local oscillation signal that is output from thefrequency synthesizer 134. Since the mixer 136 multiplies the RF signalby the local oscillation signal, the mixer 136 can output an IF signalthat is down-converted into an intermediate frequency (IF) lower than acarrier frequency, in accordance with the local oscillation signal.

By the above-described configuration, the intermediate frequencyconverting unit 122 outputs the IF signal where the frequency of the RFsignal is down-converted into the intermediate frequency.

The amplifier 124 amplifies the IF signal that is output from theintermediate frequency converting unit 122. In this case, the amplifier124 can be composed of, for example, an operational amplifier, but thepresent invention is not limited thereto.

The BPF 126 passes only a specific frequency band signal with respect tothe amplified IF signal that is output from the amplifier 124, andattenuates the other band signals. The communication device according tothe embodiment of the present invention can configure the BPF 126 usinga low-pass filter that attenuates a signal having a frequency higherthan a cutoff frequency. In this case, the signal that is processed bythe elements from the LNA 120 to the BPF 126 is an analog signal.

The A/D converter 128 discretizes a signal based on an analog IF signalthat is output from the BPF 126, and outputs a discretization signal. Inthis case, the A/D converter 128 is composed of an A/D converter thathas a resolution of N bits and sets average amplitude of a normalthermal noise as lower M bits of the A/D converter 128. Accordingly, theA/D converter 128 can prevent an output spectrum of the A/D converterfrom being saturated due to the external noise, and surely remove theexternal noise in the noise removing unit 106 that is provided at a rearstage of the A/D converter 128.

By the above-described configuration, the frequency converting unit 104can convert the frequency of the RF signal received by the communicationantenna 102 into the IF signal that is obtained by down-converting theintermediate frequency (IF), and output the discretization signal as adigital signal.

The noise removing unit 106 detects an external noise without performinga feedback operation with respect to the discretization signal andremove the external noise, based on the discretization signal that isoutput from the frequency converting unit 104. Hereinafter, an exampleof the configuration of the noise removing unit 106 according to thefirst embodiment will be described.

[Example of the Configuration of a Noise Removing Unit 106]

FIG. 7 is a diagram illustrating the configuration of a noise removingunit 106 according to a first embodiment of the present invention. InFIG. 7, the communication antenna 102 and the frequency converting unit104 are shown together. In FIG. 7, a discretization signal that has thepossibility of including an external noise output from the frequencyconverting unit 104 is shown as a discretization signal y(t), and thediscretization signal after removing the noise is shown as adiscretization signal x(t).

Referring to FIG. 7, the noise removing unit 106 includes a Fouriertransforming unit 160 (first Fourier transforming unit), a notchfrequency detecting unit 162, and a notch filter 164.

The Fourier transforming unit 160 performs a fast Fourier transform(hereinafter, also referred to as an “FFT”) on the discretization signalthat is output from the frequency converting unit 104. In addition, theFourier transforming unit 160 transmits a result of a fast Fouriertransform to the notch frequency detecting unit 162.

Further, the Fourier transforming unit 160 selectively performs, forexample, a fast Fourier transform in accordance with a control signalthat is transmitted from the MPU 144 of the demodulating unit 108. Inthis case, for example, in a single frequency noise (external noise) ora narrowband noise (external noise), such as a noise that is generatedby harmonics of a clock signal that drives a digital circuit, othernarrowband radio interference, and a power supply noise due to aswitching regulator, its frequency and amplitude is generally rarelyvaried over time. Meanwhile, a narrowband radio signal that may becomethe other external noise is not constantly transmitted and received. Forexample, in the case where an electric wave is not output, if the notchfilter 164 is operated, consumption power may be lost. Accordingly, thecommunication device 100 transmits a control signal to the Fouriertransforming unit 160 periodically such as for every 100 [msec], orduring a process routine, thereby allowing the Fourier transforming unit160 to selectively execute a fast Fourier transform. In addition, whenthe notch frequency detecting unit 162 detects that amplitude in afrequency region is at a peak of a predetermined value or more based ona result of the fast Fourier transform, for example, the communicationdevice 100 operates the notch filter 164. When the peak is not detected,the communication device 100 stops the notch filter 164. As such, thecommunication device 100 selectively executes the fast Fourier transformand selectively operates the notch filter 164 based on the result of thefast Fourier transform. As a result, the communication device 100 canremove the external noise while reducing a loss of consumption power. Asdescribed above, the communication device 100 selectively executes thefast Fourier transform, and selectively operates the notch filter 164based on the result of the fast Fourier transform. As a result, evenwhen a frequency is varied in a single frequency noise (external noise)or a narrowband noise (external noise), it is possible to remove theexternal noise. Further, the Fourier transforming unit 160 can executethe fast Fourier transform, when the discretization signal istransmitted from the frequency converting unit 104.

Further, the Fourier transforming unit 160 can be composed of adedicated fast Fourier transforming circuit, but the present inventionis not limited thereto. For example, the Fourier transforming unitaccording to the embodiment of the present invention can use (share) afast Fourier transforming circuit that is used to execute a de-spreadprocess in the demodulating unit 108.

The notch frequency detecting unit 162 detects a notch frequency f0based on the result of the fast Fourier transform that is transmittedfrom the Fourier transforming unit 160. In addition, the notch frequencydetecting unit 162 transmits a notch frequency setting signalcorresponding to the detected notch frequency f0 to the notch filter 164and allows the notch filter 164 to set the notch frequency f0.

In this case, for example, the notch frequency detecting unit 162detects a frequency having maximum amplitude among frequencies where themagnitude of amplitude in the discretization signal becomes a peak of apredetermined value or more based on the result of the fast Fouriertransform, thereby detecting the notch frequency f0, but the presentinvention is not limited thereto. The notch frequency detecting unit 162detects the notch frequency f0 as described above, thereby detecting afrequency that corresponds to a single frequency noise or a narrowbandnoise to be removed.

The notch frequency detecting unit 162 can be composed of, for example,a peak detecting circuit that has an operational amplifier or a diodeand a capacitor, but the present invention is not limited thereto. Forexample, the notch frequency detecting unit 162 may use a digital signalprocessing circuit that searches a peak, as the peak detecting circuit.Further, the notch frequency detecting unit 162 allows the peakdetecting circuit to directly transmit the notch frequency settingsignal to the notch filter 164, thereby setting the notch frequency f0to the notch filter 164, but the present invention is not limitedthereto. For example, information of the notch frequency f0 that isdetected by the peak detecting circuit is transmitted to the controlunit (not shown) or the MPU that is included in the notch frequencydetecting unit 162. The control unit (not shown) or the MPU that isincluded in the notch frequency detecting unit 162 may transmit thenotch frequency setting signal to the notch filter 164, thereby settingthe notch frequency f0. As described above, when the notch frequencydetecting unit 162 detects that amplitude in a frequency region is at apeak of a predetermined value or more based on the result of the fastFourier transform, the communication device 100 may operate the notchfilter 164. When the peak is not detected, the communication device 100may stop the notch filter 164.

The notch filter 164 applies rapid attenuation to the set notchfrequency f0, thereby removing the external noise from thediscretization signal that is output from the frequency converting unit104. Since the notch filter 164 is a filter that applies rapidattenuation to the notch frequency f0, the notch filter 164 is suitablefor mainly removing a single frequency noise or a narrowband noise. Inthis case, an example of the configuration of the notch filter 164 willbe described.

[Example of the Configuration of a Notch Filter 164]

FIG. 8 is a diagram illustrating an example of the configuration of anotch filter 164 that is included in a communication device 100according to a first embodiment of the present invention.

Referring to FIG. 8, the notch filter 164 includes a first adder 170, adelay element 172, a first multiplier 174, a second adder 176, and asecond multiplier 178.

The first adder 170 adds a discretization signal y(t) input to the notchfilter 164 and a feedback signal (which will be described in detailbelow) output from the second multiplier 178, and outputs a firstaddition signal.

The delay element 172 outputs a delay signal that is obtained bydelaying the first addition signal output from the first adder 170 by afirst period (one clock) of a sampling period.

The first multiplier 174 performs an operation represented by thefollowing Equation 1, based on the delay signal that is output from thedelay element 172, and outputs a multiplication signal. In this case, inEquation 1, “Dout1” denotes a multiplication signal, and “Din1” denotesa delay signal that is input to the first multiplier 34. Further, inEquation 1, “f0” denotes a notch frequency, and “Ts” denotes a samplingperiod. Further, in the first multiplier 174, the notch frequency f0 isset whenever the notch frequency setting signal is transmitted from thenotch frequency detecting unit 162.Dout1=Din1×e ^(j·2π·f0·Ts)  [Equation 1]

The second adder 176 subtracts the multiplication signal from the firstaddition signal. In this case, since the multiplication signal is asignal component that corresponds to the notch frequency, the secondadder 176 subtracts the multiplication signal from the first additionsignal, thereby applying rapid attenuation to the notch frequency.Accordingly, a discretization signal x(t) from which the external noisecorresponding to the notch frequency is removed is output from thesecond adder 176.

The second multiplier 178 performs an operation represented by thefollowing Equation 2, based on the multiplication signal output from thefirst multiplier 174, and outputs a feedback signal. In this case, inEquation 2, “Dout2” denotes the feedback signal, and “Din2” denotes themultiplication signal that is input to the second multiplier 38.Further, in Equation 2, “r” denotes a feedback coefficient, and when thefeedback coefficient r has a value that is approximated to 1, a notchband is narrowed. Further, the feedback coefficient r may be apreviously set fixed coefficient. For example, the feedback coefficientr may be a coefficient whose value is changed in accordance with asignal that is transmitted from the control unit (not shown).Dout2=Din2×r;(r<1)  [Equation 2]

By the configuration shown in FIG. 8, the notch filter 164 can output adiscretization signal x(t) where removed is an external noisecorresponding to the notch frequency set in accordance with the notchfrequency setting signal transmitted from the notch frequency detectingunit 162.

FIGS. 9A and 9B are diagrams illustrating an effect of when acommunication device 100 according to a first embodiment of the presentinvention includes a noise removing unit 106. In this case, FIG. 9Ashows an example of a result of a de-spread process in a demodulatingunit 108 in the case where a communication device according to anembodiment of the present invention does not include a noise removingunit 106, and FIG. 9B shows an example of a result of a de-spreadprocess in a demodulating unit 108 in the case where a communicationdevice according to an embodiment of the present invention includes anoise removing unit 106. FIGS. 9A and 9B show a result of a de-spreadprocess in a demodulating unit 108 when an A/D converter 128 outputs adiscretization signal y(t) shown in an output spectrum shown in FIG. 2.

As shown in FIGS. 9A and 9B, when the communication device according tothe embodiment of the present invention does not include the noiseremoving unit 106, that is, when the external noise is not removed, aspectrum spread signal is not detected in the demodulating unit 108(refer to FIG. 9A). Meanwhile, when the communication device accordingto the embodiment of the present invention includes the noise removingunit 106, that is, when the external noise is removed, the spectrumspread signal is detected in the demodulating unit 108 (refer to FIG.9B). In this case, the communication device 100 includes the noiseremoving unit 106, as shown in FIG. 6. Therefore, since thecommunication device 100 includes the noise removing unit 106 to removethe external noise, as shown in FIG. 9B, the communication device 100can detect the spectrum spread signal and demodulate the spectrum spreadsignal.

Referring back to FIG. 6, the constituent elements of the communicationdevice 100 will be described. The demodulating unit 108 detects aspectrum spread signal based on a discretization signal that is outputfrom the noise removing unit 106 and demodulates the detected spectrumspread signal. Hereinafter, an example of the configuration of thedemodulating unit 108 will be described.

[Example of the Configuration of a Demodulating Unit 108]

The demodulating unit 108 includes a synchronous capturing unit 140, asynchronous holding unit 142, an MPU 144, an RTC (Real Time Clock) 146,a timer 148, a memory 150, and a multiplier/divider 152.

The synchronous capturing unit 140 performs synchronous capturing of aspread code in a discretization signal that is output from the noiseremoving unit 106, based on an oscillation signal in a multiplied and/ordivided oscillation signal that is supplied from the multiplier/divider152, under the control from the MPU 144. Further, the synchronouscapturing unit 140 detects a carrier frequency in the discretizationsignal output from the noise removing unit 106 or device identificationinformation (for example, satellite number to identify a GPS satellite)indicating an external device that becomes a transmission origin of thediscretization signal, in addition to the synchronous capturing of thespread code. In addition, the synchronous capturing unit 140 transmits aphase of the detected spread code, the detected carrier frequency, andthe detected device identification information to the synchronousholding unit 142 and the MPU 144.

Further, the synchronous capturing unit 140 can be composed of, forexample, a digital matched filter using a fast Fourier transform. Inthis case, the digital matched filter may be exemplified by a digitalmatched filter using a technology that is disclosed in JP-A-2003-232844,but the present invention is not limited thereto.

The synchronous holding unit 142 synchronously holds a spread code in adiscretization signal output from the noise removing unit 106 and acarrier, based on the multiplied and/or divided oscillation signalsupplied from the multiplier/divider 152 and a variety of information (aphase of a spread code, a carrier frequency, and device identificationinformation) transmitted from the synchronous capturing unit 140, underthe control from the MPU 144. In addition, the synchronous holding unit142 demodulates data that is included in the discretization signaloutput from the noise removing unit 106, in addition to the synchronousholding. In this case, the synchronous holding unit 142 starts toexecute an initialization process on the phase of the spread code, thecarrier frequency, and the device identification information that aretransmitted from the synchronous capturing unit 140.

Further, the synchronous holding unit 142 transmits the phase of thedetected spread code, the detected carrier frequency, and the detecteddemodulated data to the MPU 144. In addition, the synchronous holdingunit 142 can execute a synchronous holding process in parallel withrespect to discretization signals that correspond to transmissionsignals transmitted from a plurality of GPS satellites (externaldevices). The synchronous holding unit 142 is exemplified by asynchronous holding unit using a technology that is disclosed inJP-A-2003-232844, but the present invention is not limited thereto.

The MPU 144 executes a process based on the phase of the spread code,the carrier frequency, and the data that are transmitted from thesynchronous holding unit 142. For example, the MPU 144 calculates alocation and a speed of the communication device 100, and executesvarious operation processes related to each GPS, for example, in orderto correct time information of the communication device 100 based ontime information of each GPS satellite (external device) that isobtained from the demodulated data.

Further, the MPU 144 can perform various control operations of thecommunication device 100 or control operations that are related to aninput/output with the external device. In the above case, the MPU 144functions as the control unit (not shown) in the communication device100.

The RTC 146 measures time based on an oscillation signal that issupplied from the XO 110. Information of the time that is measured bythe RTC 146 is substituted, for example, until the time information ofthe GPS satellite (external device) is obtained, and when the timeinformation of the GPS satellite is obtained, the MPU 144 controls thetimer 148 and appropriately corrects the time information.

The timer 148 is, for example, used to generate by the MPU 144 varioustiming signals controlling the operations of each of the elements of thecommunication device 100, or refer to time.

The memory 150 is composed of, for example, a ROM or a RAM. The ROM thatconstitutes the memory 150 records control data, such as a program or anoperation parameter used by the MPU 144. Further, the RAM primarilystores a program that is executed by the MPU 144.

The multiplier/divider 152 multiplies and/or divides an oscillationsignal that is supplied from the TCXO 12.

By the above-described configuration, the demodulating unit 108 candetect a spectrum spread signal based on the discretization signaltransmitted from the noise removing unit 106 and demodulate the spectrumspread signal.

The XO 110 generates an oscillation signal that has a predeterminedoscillation frequency, such as, for example, 32.768 kHz. In addition,the XO 110 supplies the generated oscillation signal to the RTC 146.

The TCXO 112 generates an oscillation signal whose frequency (forexample, 18.414 MHz) is different from the frequency of the oscillationsignal that is generated by the XO 110. In addition, the TCXO 112supplies the generated oscillation signal to the multiplier/divider 152or the frequency synthesizer 134.

As described above, by the configuration shown in FIG. 6, for example,the communication device 100 according to the first embodiment of thepresent invention receives the transmission signal that is transmittedfrom the external device, detects the spectrum spread signal included inthe received transmission signal, and demodulates the spectrum spreadsignal. Further, the communication device 100 includes, for example, thenoise removing unit 106 that mainly removes a narrowband noise (externalnoise) shown in FIG. 2 or a single frequency noise (external noise),which is included in the received transmission signal. In this case, thenoise removing unit 106 includes the Fourier transforming unit 160, thenotch frequency detecting unit 162, and the notch filter 164, and thenotch frequency of the notch filter 164 is set based on a result that isobtained by performing a fast Fourier transform on the discretizationsignal transmitted from the A/D converter 128. Since the noise removingunit 106 detects an external noise without performing a feedbackoperation on the discretization signal and removes the external noise,the communication device 100 does not perform an unstable operation asin the communication device 10 according to the related art.Accordingly, the communication device 100 can stably remove the externalnoise from the transmission signal where the spectrum spread signal ismodulated. Further, since the communication device 100 can stably removethe external noise, the communication device 100 can surely demodulatethe spectrum spread signal.

Further, the A/D converter 128 that is provided at the final stage ofthe analog circuit that processes an analog signal in the communicationdevice 100 is composed of an A/D converter that has a resolution of Nbits that are larger than the number of bits corresponding to averageamplitude of a normal thermal noise. In addition, the A/D converter 128sets the average amplitude of the normal thermal noise as lower M bitsof the A/D converter 128. Accordingly, the A/D converter 128 that isincluded in the communication device 100 can prevent an output spectrumof the A/D converter from being saturated due to the external noise, andsurely remove the external noise in the noise removing unit 106 that isprovided at the rear stage of the A/D converter 128.

[Modification of a Communication Device According to a First Embodiment]

In the above case, as shown in FIG. 7, the communication device 100according to the first embodiment includes the noise removing unit 106that has one notch filter 164. However, the noise removing unit that isincluded in the communication device according to the first embodimentof the present invention is not limited to the configuration shown inFIG. 7, and can include a plurality of notch filters.

FIG. 10 is a diagram illustrating the configuration of a noise removingunit 180 according to a modification of a first embodiment of thepresent invention. In FIG. 10, the communication antenna 102 and thefrequency converting unit 104 are shown together.

Referring to FIG. 10, the noise removing unit 180 includes a Fouriertransforming unit 160, a notch frequency detecting unit 182, and k (k isan integer of 2 or more) notch filters 164 a to 164 k.

Similar to the Fourier transforming unit 160 shown in FIG. 7, theFourier transforming unit 160 performs a fast Fourier transform on thediscretization signal that is output from the frequency converting unit104. In addition, the Fourier transforming unit 160 transmits the resultof the fast Fourier transform to the notch frequency detecting unit 182.

The notch frequency detecting unit 182 detects as a notch frequency afrequency where the magnitude of amplitude in the discretization signalbecomes a peak of a predetermined value or more, based on the result ofthe fast Fourier transform. In addition, the notch frequency detectingunit 182 transmits notch frequency setting signals corresponding to thedetected notch frequencies f0_1, f0_2, . . . , and f0 _(—) k to thenotch filters 164 a to 164 k, and sets the notch frequencies to thenotch filters 164 a to 164 k, respectively. In this case, the notchfrequency detecting unit 182 can set, for example, the notch frequenciesf0_1, f0_2, . . . , and f0 _(—) k in the order of the frequencies havinglarge amplitude among the detected notch frequencies, but the presentinvention is not limited thereto.

FIG. 10 shows the case where the number of notch frequencies detected bythe notch frequency detecting unit 182 is the same as the number ofnotch filters 164 k included by the noise removing unit 180, but thepresent invention is not limited thereto. For example, when the numberof notch frequencies detected by the notch frequency detecting unit 182is smaller than the number of notch filters 164 k, the notch frequencydetecting unit 182 transmits the notch frequency setting signalscorresponding to the notch frequencies that are sequentially detectedstarting from the notch filter 164 a. That is, in the above case, thenotch frequency setting signal is not transmitted to some notch filter164 k among the notch filters. At this time, the notch filter 164 kwhere the notch frequency setting signal is not transmitted sets outputsof the multipliers 174 and 178 as 0, for example. Alternatively, theoperation of the notch filter 164 k is stopped and y(t) of FIG. 8 isbypassed to x(t). Meanwhile, when the number of notch frequenciesdetected by the notch frequency detecting unit 182 is larger than thenumber of notch filters 164 k, for example, the notch frequencydetecting unit 182 transmits the notch frequency setting signals in theorder of the notch frequencies having large amplitude among the detectednotch frequencies. In the above case, the notch frequency setting signalthat corresponds to a portion of the notch frequencies detected by thenotch frequency detecting unit 182 are not transmitted.

Each of the notch filters 164 a to 164 k has the same configuration asthe notch filter 164 shown in FIG. 8, and removes the external noisethat corresponds to the notch frequency set in accordance with thediscretization signal output from the frequency converting unit 104.

For example, by the configuration shown in FIG. 10, even when aplurality of single frequency noises or narrowband noises (externalnoises) where a noise band is narrow are included in the discretizationsignal output from the frequency converting unit 104, the noise removingunit 180 according to the modification of the first embodiment canremove the plurality of external noises.

The communication device according to the modification of the firstembodiment includes, for example, the noise removing unit 180 shown inFIG. 10. As a result, even when the plurality of single frequency noisesor narrowband noises (external noises) where a noise band is narrow areincluded in the discretization signal output from the frequencyconverting unit 104, the communication device can remove the pluralityof external noises. Accordingly, the communication device according tothe modification of the first embodiment can remove a large amount ofexternal noises, as compared with the communication device 100 accordingto the first embodiment shown in FIGS. 6 and 7.

Further, in the communication device according to the modification ofthe first embodiment, the configuration of the noise removing unit isdifferent from that of the noise removing unit in the communicationdevice 100 according to the first embodiment shown in FIGS. 6 and 7.However, the communication device according to the modification of thefirst embodiment can detect a plurality of external noises withoutperforming a feedback operation on the discretization signal, and removethe plurality of external noises. Accordingly, the communication deviceaccording to the modification of the first embodiment can achieve thesame effect as the communication device 100 according to the firstembodiment.

(Program According to a First Embodiment)

The external noise can be stably removed from the transmission signalwhere the spectrum spread signal is modulated, by using a program thatallows a computer to function as the noise removing unit 106 of thecommunication device 100 according to the first embodiment.

(Noise Removing Method According to a First Embodiment)

Next, a noise removing method according to the first embodiment will bedescribed. FIG. 11 is a flowchart illustrating a noise removing methodaccording to a first embodiment of the present invention. In thedescription below, the noise removing method shown in FIG. 11 isexecuted by the communication device 100 (specifically, noise removingunit 106).

The communication device 100 performs a fast Fourier transform on thediscretization signal (S100). In this case, the communication device 100can periodically or randomly execute a process of Step S100. Further,the process may be executed whenever the discretization signal is input.

Based on a result of the fast Fourier transform in Step S100, thecommunication device 100 detects a peak frequency (S102). In this case,the communication device 100 detects, for example, a frequency havingmaximum amplitude as a peak frequency among frequencies having amplitudeof a predetermined value or more, based on the result of the fastFourier transform, but the present invention is not limited thereto.

Based on the peak frequency that is detected in Step S102, thecommunication device 100 outputs a discretization signal where afrequency component corresponding to the peak frequency is attenuated(S104). In this case, the communication device 100 includes the notchfilter as the noise removing unit, and can execute the process of StepS104 by setting the peak frequency detected in Step S102 as the notchfrequency of the notch filter.

Using the method shown in FIG. 11, the communication device 100 candetect the external noise without performing a feedback operation on thediscretization signal, based on the discretization signal having thepossibility of including the external noise, and output thediscretization signal where the external noise is removed.

(Second Embodiment)

In the above case, as the communication device according to the firstembodiment of the present invention, the description has been given tothe communication device 100 including the noise removing unit thatmainly removes a single frequency noise (external noise) or a narrowbandnoise (external noise) shown in FIG. 2. However, the external noise thatis removed by the communication device according to the embodiment ofthe present invention is not limited to the noise of the singlefrequency or the narrowband noise shown in FIG. 2. For example, thecommunication device according to the embodiment of the presentinvention can remove a broadband noise having a broad noise band that isshown in FIG. 12 (external noise. Here, FIG. 12 shows an FM wave).Accordingly, as a communication device according to a second embodimentof the present invention, the description is given to a communicationdevice that includes a noise removing unit that can remove the broadbandnoise (external noise).

[Noise Removing Approach According to a Second Embodiment]

When the communication device according to the embodiment of the presentinvention communicates with the GPS satellite (external device), S/(N+1)becomes significantly smaller than 0 [dB]. Further, in power of adiscretization signal in an ideal state (hereinafter, referred to as“ideal discretization signal”) where the external noise is not included,a thermal noise approximates to 100%, and a statistical property of thethermal noise itself is constant.

Further, if the ideal discretization signal is set as x(t), the externalnoise is set as n(t), and the discretization signal based on thereceived transmission signal is set as y(t), a relationship between thediscretization signal y(t), the ideal discretization signal x(t), andthe external noise n(t) is represented by the following Equation 3.y(t)=x(t)+n(t)  [Equation 3]

Accordingly, the communication device according to the second embodimentuses a Wiener filter that minimizes a minimum mean squared error toobtain a discretization signal x′(t) where a mean squared error of theideal discretization signal x(t) represented in Equation 3 is minimized,thereby removing an external noise from the discretization signal y(t).In this case, the communication device according to the secondembodiment uses the results Y(f), X(f), and N(f) that are obtained byperforming a Fourier transform on the discretization signal y(t), theideal discretization signal x(t), and the external noise n(t), and thepower spectrums Py(f), Px(f), and Pn(f), thereby obtaining thediscretization signal x′(t), which will be specifically described below.

Since the ideal discretization signal x(t) mainly includes a thermalnoise, if dispersion of amplitude is assumed as σx², σx² becomes aconstant value without depending on a frequency. Accordingly, the powerspectrum Px(f) of the ideal discretization signal x(t) can beapproximately represented by the following Equation 4. In this case, AFdenotes a bandwidth (for example, 2 MHz) of the discretization signal.Further, σx² can be determined, for example, by observing the samesignal as a transmission signal in a state where the communicationantenna is not connected, but the present invention is not limitedthereto. That is, Px(f) in Equation 4 denotes power per unit frequency(for example, 1 MHz). In the description below, since the power spectrumPx(f) of the ideal discretization signal x(t) denotes power per unitfrequency, the power spectrum Px(f) may be referred to as “referencepower”.Px(f)=σx ² /ΔF  [Equation 4]

In addition, the power spectrum Pn(f) of the external noise can berepresented by the following Equation 5 from Equation 3.Pn(f)=Py(f)−Px(f)  [Equation 5]

Further, if Equations 4 and 5 and Wiener filter logic are applied, aresult W(f) of a Fourier transform of the Wiener filter can berepresented by the following Equation 6.

$\begin{matrix}\begin{matrix}{{W(f)} = {1/\left\{ {1 + {{{Pn}(f)}/{{Px}(f)}}} \right\}}} \\{= {{{Px}(f)}/{{Py}(f)}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

The communication device according to the second embodiment uses a fastFourier transforming circuit where a point number N_(FET) is set, forexample, as power-of-two, thereby previously setting the reference powerPx(n) by Equation 7, based on Equation 4.Px(n)=σx ² /Δf  [Equation 7]

The communication device according to the second embodiment uses thefast Fourier transforming circuit where a point number N_(FET) is set,for example, as power-of-two, thereby obtaining a power spectrum of thediscretization signal y(t) based on the discretization signal y(t) fromthe following Equation 8. In this case, in Equation 8, n is an integerof n=0 to N_(FET)−1, and N_(FET) ² denotes a correction coefficient thatcorrects an output of the fast Fourier transforming circuit using apoint number. Further, in Equation 8, Δf denotes a resolution of thefast Fourier transforming circuit. If a sampling frequency Fs is used,Δf is represented as Δf=Fs/N_(FET). For Example, when it is assumed thatthe sampling frequency Fs is 16 [MHz] and the point number N_(FET) is64, Δf becomes 250 [kHz].Py(n)=Y(n)² /N _(FET) ² /Δf  [Equation 8]

The communication device according to the second embodiment uses theWiener filter that has a relationship represented in Equation 6, therebyobtaining a discretization signal x′(t) where a mean squared error ofthe ideal discretization signal x(t) is minimized, based on thediscretization signal y(t) (specifically, Py(n) represented in Equation8). Further, the communication device according to the second embodimentdoes not need to perform a feedback operation on the discretizationsignal y(t) in order to detect the external noise, as in thecommunication device 10 according to the related art. Accordingly, thecommunication device according to the second embodiment can detect theexternal noise without performing a feedback operation on thediscretization signal y(t) and remove the external noise from thediscretization signal y(t).

The communication device 100 according to the first embodiment uses anotch filter that applies rapid attenuation to a specific frequency(notch frequency) and removes an external noise. Thus, the communicationdevice 100 is suitable, for example, for mainly removing a singlefrequency noise or a narrowband noise (external noise) where a noiseband is narrow. Meanwhile, since the communication device according tothe second embodiment uses the Wiener filter as a type of adaptivefilter to remove the external noise, it is possible to effectivelyremove not only the noise of the single frequency or the narrowbandnoise where the noise band is narrow but also a biased broadband noisewhere a frequency is temporally varied, which is shown in FIG. 12.

Further, in the above case, logic of the Wiener filter in the frequencyregion is applied as the noise removing approach according to the secondembodiment, but the noise removing approach in the communication deviceaccording to the second embodiment is not limited thereto. For example,the communication device according to the second embodiment can removethe broadband noise by using a method that adaptively controlscoefficients of a finite impulse response (FIR) filter and a finiteimpulse response tap, by a Wiener-hoff equation that is a solution in atime domain.

Hereinafter, the communication device 200 according to the secondembodiment of the present invention will be described. In this case, thecommunication device 200 and the communication device 100 according tothe first embodiment are different from each other in a noise removingapproach. Accordingly, the communication device 200 is different fromthe communication device 100 in the configuration of the noise removingunit that removes the external noise, but the other configurationthereof is the same as the configuration of the communication device100. Accordingly, in the description below, the configuration of a noiseremoving unit 202 of the communication device 200 is described and theother configuration is not described.

[Example of the Configuration of a Noise Removing Unit 202]

FIG. 13 is a diagram illustrating an example of the configuration of anoise removing unit 202 according to a second embodiment of the presentinvention. In this case, in FIG. 13, the communication antenna 102 andthe frequency converting unit 104 are shown together. Further, in FIG.13, a discretization signal that has the possibility of including theexternal noise output from the frequency converting unit 104 is shown asa discretization signal y(t) and the discretization signal afterremoving the noise is shown as a discretization signal x′(t). In thiscase, the discretization signal x′(t) that is output from the noiseremoving unit 202 is transmitted to the demodulating unit 108.

Referring to FIG. 13, the noise removing unit 202 that functions as anadaptive filter includes a Fourier transforming unit 204 (second Fouriertransforming unit) and a Wiener filter 206 (first Wiener filter).

The Fourier transforming unit 204 performs a fast Fourier transform onthe discretization signal y(t) output from the A/D converter 128 of thefrequency converting unit 104, and derives a power spectrumPy(n)=Y(n)²/N_(FET) ²/Δf based on the result Y(n) of the fast Fouriertransform and outputs the power spectrum. In this case, the Fouriertransforming unit 204 can include, for example, a fast Fouriertransforming circuit, and an operation circuit that derives the powerspectrum Py(n) based on the result of the fast Fourier transform, butthe present invention is not limited thereto. Further, in thecommunication device 200, based on the result of the fast Fouriertransform in the Fourier transforming unit 204, for example, the MPU 144of the demodulating unit 108 or the control unit (not shown) can derivethe power spectrum Py(n).

The Wiener filter 206 outputs the discretization signal x′(t) where amean squared error of the ideal discretization signal x(t) is minimized,based on the discretization signal y(t) output from the A/D converter128, the power spectrum Py(n) output from the Fourier transforming unit204, and the reference power Px(n). In this case, a value of thereference power Px(n) is transmitted, for example, by the MPU 144 of thedemodulating unit 108 or the control unit (not shown), but the presentinvention is not limited thereto.

By the configuration that is shown in FIG. 13, for example, the noiseremoving unit 202 can output the discretization signal x′(t) where amean squared error of the ideal discretization signal x(t) is minimized,that is, the discretization signal where the external noise is removed,based on the discretization signal y(t) output from the A/D converter128.

As described above, the communication device 200 according to the secondembodiment of the present invention basically has the same configurationas the communication device 100 according to the first embodiment shownin FIG. 6, receives the transmission signal transmitted from theexternal device, detects the spectrum spread signal included in thereceived transmission signal, and demodulates the spectrum spreadsignal. Further, the communication device 200 has the Wiener filter, andincludes the noise removing unit 202 that functions, for example, as theadaptive filter to remove the narrowband noise or the noise of thesingle frequency (external noise) shown in FIG. 2, or the broadbandnoise (external noise) shown in FIG. 12. The noise removing unit 202outputs the discretization signal where the mean squared error of theideal discretization signal is minimized, that is, the discretizationsignal where the external noise is removed, based on the discretizationsignal transmitted from the A/D converter 128, the power spectrum basedon the result of the fast Fourier transform with respect to thediscretization signal, and the power spectrum of the idealdiscretization signal. In this case, since the noise removing unit 202does not need to perform a feedback operation on the discretizationsignal in order to detect the external noise, the communication devicedoes not perform an unstable operation, as in the communication device10 according to the related art. Accordingly, the communication device200 can stably remove the external noise from the transmission signalwhere the spectrum spread signal is modulated. Further, since thecommunication device 200 can stably remove the external noise, thecommunication device 200 can surely demodulate the spectrum spreadsignal.

Further, similar to the communication device 100 according to the firstembodiment, in the communication device 200, the A/D converter 128 thatis provided at the final stage of the analog circuit to process ananalog signal is composed of an A/D converter that has a resolution of Nbits that are larger than the number of bits corresponding to averageamplitude of a normal thermal noise. In addition, the A/D converter 128sets the average amplitude of the normal thermal noise as lower M bitsof the A/D converter 128. Accordingly, the A/D converter 128 that isincluded in the communication device 200 can prevent the output spectrumof the A/D converter from being saturated due to the external noise, andsurely remove the external noise in the noise removing unit 202 that isprovided at the rear stage of the A/D converter 128.

(Program According to a Second Embodiment)

The external noise can be stably removed from the transmission signalwhere the spectrum spread signal is modulated, by using a program thatallows a computer to function as the noise removing unit 202 of thecommunication device 200 according to the second embodiment.

(Noise removing method according to a second embodiment)

Next, a noise removing method according to a second embodiment will bedescribed. FIG. 14 is a flowchart illustrating a noise removing methodaccording to a second embodiment of the present invention. In thedescription below, the noise removing method shown in FIG. 14 isexecuted by the communication device 200 (specifically, noise removingunit 202).

The communication device 200 performs a fast Fourier transform on thediscretization signal, and derives a power spectrum based on the resultof the fast Fourier transform (S200).

If the power spectrum is derived in Step S200, the communication device200 outputs a discretization signal where a mean squared error withrespect to an ideal discretization signal is minimized, based on thediscretization signal, the power spectrum derived in Step S200, and thereference power (S202). In this case, the communication device 200includes the Wiener filter as the noise removing unit, thereby executinga process of Step S202.

Using the method shown in FIG. 14, the communication device 200 candetect the external noise without performing a feedback operation on thediscretization signal, based on the discretization signal having thepossibility of including the external noise, and output thediscretization signal where the external noise is removed.

(Third Embodiment)

In the above case, as the communication device according to the secondembodiment of the present invention, the description has been given tothe communication device 200 including the noise removing unit 202 thatfunctions as the adaptive filter capable of removing a broadband noiseand has the Wiener filter. However, the configuration of the noiseremoving unit that is included by the communication device according tothe embodiment of the present invention and can remove the broadbandnoise is not limited to the configuration of the noise removing unit 202shown in FIG. 13. Accordingly, a communication device 300 according to athird embodiment of the present invention that is another embodiment ofthe communication device that includes the noise removing unit capableof removing a broadband noise will be described.

In this case, the communication device 300 and the communication device200 according to the second embodiment are different from each other inthe configuration of the noise removing unit, and the otherconfiguration thereof is the same as the configurations of thecommunication device 100 according to the first embodiment and thecommunication device 200 according to the second embodiment.Accordingly, in the description below, the configuration of a noiseremoving unit 302 of the communication device 300 is described and theother configuration is not described.

[Example of the Configuration of a Noise Removing Unit 302]

FIG. 15 is a diagram illustrating an example of the configuration of anoise removing unit 302 according to a third embodiment of the presentinvention. In this case, in FIG. 15, the communication antenna 102 andthe frequency converting unit 104 are shown together. Further, in FIG.15, a discretization signal that has the possibility of including anexternal noise output from the frequency converting unit 104 is shown asa discretization signal y(t) and the discretization signal afterremoving the noise is shown as a discretization signal x′(t). In thiscase, the discretization signal x′(t) that is output from the noiseremoving unit 502 is transmitted to the demodulating unit 108.

Referring to FIG. 15, the noise removing unit 302 includes aserial/parallel converting unit 304, a Fourier transforming unit 306(third Fourier transforming unit), a determining unit 308 (firstdetermining unit), a Wiener filter 206 (hereinafter, it may also bereferred to as a “Wiener filter operation unit 206”; second Wienerfilter), an inversed Fourier transforming unit 310, and aparallel/serial converting unit 312.

The serial/parallel converting unit 304 converts the discretizationsignal y(t) output from the A/D converter 128 of the frequencyconverting unit 104 from serial data to parallel data. In this case, theserial/parallel converting unit 304 can be configured, for example, byusing a shift register, but the present invention is not limitedthereto. Further, when the discretization signal y(t) is parallel data,the serial/parallel converting unit 304 does not execute a convertingprocess.

The Fourier transforming unit 306 performs a fast Fourier transform on adiscretization signal y(t), which is output from the serial/parallelconverting unit 304 and converted into parallel data.

The determining unit 308 derives a power spectrum Py(n)=Y(n)²/N_(FET)²/Δf of the discretization signal, based on the result Y(n) of the fastFourier transform transmitted from the Fourier transforming unit 306. Inaddition, the determining unit 308 compares the magnitudes of thederived power spectrum Py(n) and the reference power Px(n) andselectively changes an output destination of the result Y(n) of the fastFourier transform in accordance with the compared result.

[1] Case of Py(n)>Px(n)

When the magnitude of the power spectrum Py(n) is larger than themagnitude of the reference power Px(n), this means that the externalnoise is included in the discretization signal y(t). Accordingly, in theabove case, the determining unit 308 outputs the result Y(n) of the fastFourier transform and the power spectrum Py(n) to the Wiener filteroperation unit 206.

[2] Case of Py(n)≦Px(n)

When the magnitude of the power spectrum Py(n) is smaller than or equalto the magnitude of the reference power Px(n), this means that thepossibility of including the external noise in the discretization signaly(t) is low and a level of the external noise is not higher than a levelof the thermal noise. Thus, the possibility of causing an issue in thedemodulation of the spectrum signal is low. Accordingly, in the abovecase, the determining unit 308 outputs the result Y(n) of the fastFourier transform to the inversed Fourier transforming unit 310, not theWiener filter operation unit 206 that removes the external noise.

In this case, the determining unit 308 can be configured, for example,by using an operation circuit that derives the power spectrum Py(n)based on the result Y(n) of the fast Fourier transform or a digitalcomparator that compares the reference power Px(n) and the powerspectrum Py(n), but the present invention is not limited thereto.

When the result Y(n) of the fast Fourier transform and the powerspectrum Py(n) are transmitted from the determining unit 308, the Wienerfilter operation unit 206 performs an operation shown in the followingEquation 9, and outputs the fast Fourier transformed result X′(n) thatcorresponds to the discretization signal x′(t) where a mean squarederror of the ideal discretization signal x(t) is minimized. In thiscase, the Wiener filter operation unit 206 can have the sameconfiguration as the Wiener filter operation unit 206 according to thesecond embodiment shown in FIG. 13.X′(n)=W(n)·Y(n)=Y(n)·{Px(n)/Py(n)}  [Equation 9]

The inversed Fourier transforming unit 310 performs an inversed fastFourier transform (hereinafter, it may also be referred to as an “IFFT”)with respect to the result Y(n) of the fast Fourier transform outputfrom the determining unit 308 or the fast Fourier transformed resultX′(n) output from the Wiener filter operation unit 206. In this case,the result Y(n) of the fast Fourier transform output from thedetermining unit 308 to the inversed Fourier transforming unit 310corresponds to the discretization signal where the possibility ofincluding the external noise is low, and the fast Fourier transformedresult X′(n) output from the Wiener filter operation unit 206corresponds to the discretization signal where the external noise isremoved in the Wiener filter operation unit 206. Accordingly, theinversed Fourier transforming unit 310 outputs the same discretizationsignal as the ideal discretization signal or a discretization signalthat includes an external noise to a degree to which the discretizationsignal can be regarded as the same signal as the ideal discretizationsignal.

By the configuration shown in FIG. 15, for example, the noise removingunit 302 can output the discretization signal where the external noiseis removed (or the external noise is not included), based on thediscretization signal y(t) that is output from the A/D converter 128.Accordingly, the noise removing unit 302 can function as an adaptivefilter that can remove the external noise.

Further, as shown in FIG. 15, the noise removing unit 302 includes theFourier transforming unit 306 and the inversed Fourier transforming unit310, and performs the fast Fourier transform (FFT) and the inversed fastFourier transform (IFFT). In this case, for example, when the Fouriertransforming unit 306 and the inversed Fourier transforming unit 310each include a fast Fourier transforming unit where the point numberN_(FET) is set as 64, each fast Fourier transforming circuit operatesthe discretization signals y(t) for every 64 signals. At this time, asshown in FIG. 16A for example, if the operation is simply performed forevery 64 signals, due to a cyclic property of the fast Fourier transform(FFT) and the inversed fast Fourier transform (IFFT), jointed signalsfor every 64 signals may become significantly discontinuous. Asdescribed above, when the signals become significantly discontinuous, adiscontinuous portion of the signals becomes a new noise.

Accordingly, as shown in FIG. 16B for example, in the noise removingunit 302, the Fourier transforming unit 306 overlaps the discretizationsignals y(t) in the fast Fourier transforming circuit where the pointnumber N_(FET) is set as 64 by 64/2=32 and performs a fast Fouriertransform (FFT) on the signals. In addition, the inversed Fouriertransforming unit 310 executes a smoothing process that outputs onlyapproximately central 32 signals among 64 signals as the result x′(t) ofthe inversed fast Fourier transform (IFFT), in the fast Fouriertransforming circuit where the point number N_(FET) is set as 64.

Since the noise removing unit 302 can hold continuity of signals byoperating the Fourier transforming unit 306 and the inversed Fouriertransforming unit 310 as described above, it is possible to preventdiscontinuity of signals from being generated in the fast Fouriertransform (FFT) and the inversed fast Fourier transform (IFFT), that is,a new noise from being generated. Further, the fast Fourier transformingprocess and the inversed fast Fourier transforming process in the noiseremoving unit according to the third embodiment of the present inventionare not limited to the above examples. For example, the noise removingunit according to the third embodiment of the present invention canexecute a fast Fourier transforming process and an inversed fast Fouriertransforming process, as shown in FIG. 16A.

As described above, the communication device 300 according to the thirdembodiment of the present invention basically has the same configurationas the communication device 100 according to the first embodiment shownin FIG. 6, and receives the transmission signal transmitted from theexternal device, detects the spectrum spread signal included in thereceived transmission signal, and demodulates the spectrum spreadsignal. Further, the communication device 300 includes the noiseremoving unit 302 that determines a degree to which the external noiseis included based on the discretization signal and selectively removesthe external noise in accordance with the determined result. In thiscase, similar to the noise removing unit 202 according to the secondembodiment shown in FIG. 13, the noise removing unit 302 uses the Wienerfilter to remove the external noise. That is, since the noise removingunit 302 does not need to perform a feedback operation on thediscretization signal in order to detect the external noise, thecommunication device does not perform an unstable operation, as in thecommunication device 10 according to the related art. Accordingly, thecommunication device 300 can stably remove the external noise from thetransmission signal where the spectrum spread signal is modulated.Further, since the communication device 300 can stably remove theexternal noise, the communication device 300 can surely demodulate thespectrum spread signal.

Further, similar to the communication device 100 according to the firstembodiment, in the communication device 300, the A/D converter 128 thatis provided at the final stage of the analog circuit to process ananalog signal is composed of an A/D converter that has a resolution of Nbits that are larger than the number of bits corresponding to averageamplitude of a normal thermal noise. In addition, the A/D converter 128sets the average amplitude of the normal thermal noise as lower M bitsof the A/D converter 128. Accordingly, the A/D converter 128 that isincluded in the communication device 300 can prevent the output spectrumof the A/D converter from being saturated due to the external noise, andsurely remove the external noise in the noise removing unit 302 that isprovided at the rear stage of the A/D converter 128.

(Program According to a Third Embodiment)

The external noise can be stably removed from the transmission signalwhere the spectrum spread signal is modulated, by using a program thatallows a computer to function as the noise removing unit 302 of thecommunication device 300 according to the third embodiment.

(Noise Removing Method According to a Third Embodiment)

Next, a noise removing method according to a third embodiment will bedescribed. FIG. 17 is a flowchart illustrating a noise removing methodaccording to a third embodiment of the present invention. In thedescription below, the noise removing method shown in FIG. 17 isexecuted by the communication device 300 (specifically, noise removingunit 302).

Similar to Step S100 of FIG. 11, the communication device 300 performs afast Fourier transform on the discretization signal y(t) (S300). Inaddition, similar to Step S200 of FIG. 14, the communication device 300derives a power spectrum Py(n) based on the result Y(n) of the fastFourier transform in Step S300 (S302).

If the power spectrum Py(n) is derived in Step S302, the communicationdevice 300 determines whether the power spectrum Py(n) derived in StepS302 is larger than the reference power Px(n) (S304).

When it is determined that the power spectrum Py(n) is larger than thereference power Px(n) in Step S304, the communication device 300 outputsthe result X′(n) of the fast Fourier transform that corresponds to thediscretization signal that minimizes a mean squared error with respectto the ideal discretization signal, based on the result Y(n) of the fastFourier transform in Step S300, the power spectrum Py(n) derived in StepS302, and the reference power Px(n) (S306). In this case, thecommunication device 300 includes the noise removing unit that has aWiener filter operation function, thereby executing the process of StepS306.

If the result X′(n) of the fast Fourier transform is output in StepS306, the communication device 300 performs an inversed Fouriertransform on the result X′(n) of the fast Fourier transform (S310). Inthe above case, the communication device 300 can output thediscretization signal x′(t) that minimizes a mean squared error withrespect to the ideal discretization signal.

Further, when it is determined in Step S304 that the power spectrumPy(n) is not larger than the reference power Px(n), the communicationdevice 300 outputs the result Y(n) of the fast Fourier transform in StepS300 (S308). In addition, the communication device 300 performs aninversed Fourier transform on the result Y(n) of the fast Fouriertransform (S310). In the above case, the communication device 300 canoutput the discretization signal x′(t) that does not include an externalnoise.

Using the method shown in FIG. 17, the communication device 300 candetect the external noise without performing a feedback operation on thediscretization signal, based on the discretization signal having thepossibility of including the external noise, and output thediscretization signal where the external noise is removed.

(Fourth Embodiment)

In the above case, as the communication devices according to the secondand third embodiments of the present invention, the description has beengiven to the communication devices including the noise removing unitthat has the Fourier transforming unit or the inversed Fouriertransforming unit, thereby removing the broadband noise. However, theconfiguration of the noise removing unit that is included in thecommunication device according to the embodiment of the presentinvention and can remove the broadband noise is not limited to theconfiguration that has the Fourier transforming unit or the inversedFourier transforming unit, as shown in FIG. 13 or FIG. 15. Accordingly,as a communication device according to the fourth embodiment, acommunication device 400 including a noise removing unit 402 that canremove a broadband noise without using a fast Fourier transform will bedescribed.

[Noise Removing Approach According to a Fourth Embodiment]

In the noise removing unit 302 that performs a fast Fourier transform(FFT) and an inversed fast Fourier transform (IFFT) shown in FIG. 15, ifa sampling frequency is set as Fs, frequencies from −Fs/2 to Fs/2 (forexample, in the case of Fs=16 MHz, −8 MHz to 8 MHz) are equally handled.However, in the communication device according to the embodiment of thepresent invention, as shown in FIG. 6 for example, in the frequencyconverting unit 104, a band is restricted in the various filters, suchas the BPF 130 or BPF 126. In this case, when the communication deviceaccording to the embodiment of the present invention receives a GPSsignal, as a representative example of a signal band of an IF signal inthe case where an intermediate frequency is zero, a band of −2 MHz to 2MHz is exemplified. Accordingly, in the communication device accordingto the embodiment of the present invention, if a Wiener filter operationshown in Equation 9 is performed, for example, with respect to all thefrequencies in a range of −Fs/2 to Fs/2, this corresponds to anoverspec.

Accordingly, if the communication device according to the embodiment ofthe present invention performs the Wiener filter operation by a portionthat corresponds to the IF signal band among −Fs/2 to Fs/2, the processcan be efficiently executed. However, even in the above case, since thepoint number N_(FET) of the fast Fourier transforming circuit that isincluded in the noise removing unit does not vary, an operation amountin the fast Fourier transforming circuit does not greatly vary. Further,when an overlapping process is executed to prevent discontinuity due tocircularity in the inversed fast Fourier transform (IFFT) and the fastFourier transform (FFT) shown in FIG. 16B, the operation amount in thefast Fourier transforming circuit may be further increased.

Accordingly, the communication device 400 according to the fourthembodiment of the present invention includes a noise removing unit 402that has a plurality of BPFs whose central frequencies are differentfrom each other by the amount that can cover an IF signal band andperforms a Wiener filter operation on each output of the BPFs. In thiscase, the noise removing unit 402 uses, for example, a frequencysampling filter as the configuration that has the plurality of BPFswhose central frequencies are different from each other. By the aboveconfiguration, the communication device 400 can include the noiseremoving unit 402 that includes minimum components needed to demodulatea targeted spectrum spread signal. Further, since the communicationdevice 400 removes an external noise without using the fast Fouriertransforming circuit, it is possible to reduce a circuit scale of thenoise removing unit, as compared with the configuration using the fastFourier transforming circuit. Further, it is possible to prevent thediscontinuity from being generated due to the circularity in the fastFourier transform (FFT) and the inversed fast Fourier transform (IFFT).

Hereinafter, the configuration of the communication device 400 accordingto the fourth embodiment of the present invention will be described. Inthis case, the communication device 400 is different from thecommunication devices according to the second and third embodiments inthe configuration of the noise removing unit, but the otherconfiguration thereof is the same as the configurations of thecommunication devices according to the first to third embodiments.Accordingly, in the description below, the configuration of the noiseremoving unit 402 of the communication device 400 is described and theother configuration thereof is not described.

[Example of the Configuration of a Noise Removing Unit 402]

FIG. 18 is a diagram illustrating an example of the configuration of anoise removing unit 402 according to a fourth embodiment of the presentinvention. In this case, in FIG. 18, the communication antenna 102 andthe frequency converting unit 104 are shown together. Further, in FIG.18, a discretization signal that has the possibility of including anexternal noise output from the frequency converting unit 104 is shown asa discretization signal y(t) and the discretization signal afterremoving the noise is shown as a discretization signal x′(t). In thiscase, the discretization signal x′(t) that is output from the noiseremoving unit 502 is transmitted to the demodulating unit 108.

Referring to FIG. 18, the noise removing unit 402 includes a BPF commonunit 404, BPF individual units 406 a to 406 g, adaptive filters 408 a to408 g (second adaptive filters), and a synthesizing unit 410.

In this case, in the noise removing unit 402, the BPF common unit 404,the BPF individual units 406 a to 406 g, and the synthesizing unit 410constitute a frequency sampling filter. Further, the BPF common unit 404and the BPF individual unit 406 a constitute one BPF. In the same way,the BPF common unit 404 and the BPF individual unit 406 b, . . . , andthe BPF common unit 404 and the BPF individual unit 406 g eachconstitute one BPF. Accordingly, the noise removing unit 402 shown inFIG. 18 is configured to include the seven BPFs. Further, thepredetermined frequency band signals (detection signals) that are outputfrom the individual BPFs are synthesized by the synthesizing unit 410,thereby configuring the frequency sampling filter that includes theseven BPFs.

FIG. 19 is a diagram illustrating an example of an output characteristicof a frequency sampling filter that is included in a noise removing unit402 according to a fourth embodiment of the present invention. In thiscase, FIG. 19 shows an example of adaptive filter operation units 408 ato 408 g where the conditions W(f)=1, the point number Nd=64, a chiprate of a transmission signal fo=1.023 MHz, and a sampling frequencyfs=16fo are set. At this time, a central frequency of each BPF thatconstitutes a frequency sampling filter in the noise removing unit 402is zero, ±fs/64, ±2fs/64, or ±3fs/64, and a frequency width of a mainlobe of each BPF becomes fs/64. FIG. 19 shows the case where the entireband width of the frequency sampling filter is exemplified as 7fs/64≈1.8MHz, but the frequency sampling filter that is included in the noiseremoving unit 402 is not limited thereto. For example, the noiseremoving unit 402 can arbitrarily set a bandwidth of the frequencysampling filter depending on the point number Nd, the chip rate fo ofthe transmission signal, the sampling frequency fs, and the number ofBPFs.

Each of the adaptive filter operation units 408 a to 408 g removes anexternal noise based on the discretization signal (detection signal)that is output from each of the BPFs whose central frequencies aredifferent from each other. Hereinafter, the configuration of theadaptive filter that is included in the noise removing unit 404 will bedescribed by exemplifying the adaptive filter 408 a. Further, since eachof the adaptive filter operation units 408 b to 408 g can include thesame configuration as the adaptive filter operation unit 408 a, thedetailed description thereof will be omitted.

[Example of the Configuration of an Adaptive Filter Operation Unit 408a]

FIG. 20 is a diagram illustrating an example of the configuration of anadaptive filter operation unit 408 a that is included in a noiseremoving unit 402 according to a fourth embodiment of the presentinvention.

Referring to FIG. 20, the adaptive filter operation unit 408 a includesa determining unit 308 a (second determining unit), a Wiener filteroperation unit 206 a (third Wiener filter), and an OR operation unit 420a.

The determining unit 308 a derives a power spectrum Py3(t)=y3(t)²/N_(d)² of a discretization signal, based on a discretization signal y3(t)that is output from the BPF individual unit 406 a. In addition, thedetermining unit 308 a compares the magnitudes of the derived powerspectrum Py3(t) and the reference power Px(t)=σx²/Δf, and selectivelychanges an output destination of the discretization signal y3(t) inaccordance with the compared result. In this case, the determining unit308 a can have the same configuration as the determining unit 308according to the third embodiment that is shown in FIG. 15.

[1] Case of Py3(t)>Px(t)

When the magnitude of the power spectrum Py3(t) is larger than themagnitude of the reference power Px(t), this means that the externalnoise is included in the discretization signal y3(t). Accordingly, inthe above case, the determining unit 308 a outputs the discretizationsignal y3(t) and the power spectrum Py3(t) to the Wiener filteroperation unit 206 a.

[2] Case of Py3(t)≦Px(t)

When the magnitude of the power spectrum Py3(t) is smaller than or equalto the magnitude of the reference power Px(t), this means that thepossibility of including the external noise in the discretization signaly3(t) is low and the level of the external noise is lower than the levelof the thermal noise. As a result, the possibility of causing an issuein the demodulation of the spectrum signal is low. Accordingly, in theabove case, the determining unit 308 a outputs the discretization signaly3(t) to the OR operation unit 420 a, not the Wiener filter operationunit 206 a that removes the external noise.

When the discretization signal y3(t) and the power spectrum Py3(t) aretransmitted from the determining unit 308 a, the Wiener filter operationunit 206 a performs an operation shown in the following Equation 10, andoutputs the discretization signal x′3(t) where a mean squared error ofthe ideal discretization signal in the discretization signal y3(t) isminimized. In this case, the Wiener filter operation unit 206 a can havethe same configuration as the Wiener filter operation unit 206 accordingto the second embodiment that is shown in FIG. 13.X′3(t)=W(t)·y3(t)=y3(t)·{Px(t)/Py3(t)}  [Equation 10]

When receiving the discretization signal y3(t) output from thedetermining unit 308 a or the discretization signal x′3(t) output fromthe Wiener filter operation unit 206 a, the OR operation unit 420 aoutputs the received discretization signals. In this case, thediscretization signal y3(t) that is output from the determining unit 308a to the OR operation unit 420 a corresponds to the discretizationsignal where the possibility of including the external noise is low, andthe discretization signal x′3(t) output from the Wiener filter operationunit 206 a corresponds to the discretization signal where the externalnoise is removed. Accordingly, the OR operation unit 420 a outputs thesame discretization signal as the ideal discretization signal in thediscretization signal y3(t) or a discretization signal that includes anexternal noise to a degree to which the discretization signal can beregarded as the same signal as the ideal discretization signal. Further,for example, the OR operation unit 420 a can be composed of an ORcircuit, but the present invention is not limited thereto.

By the configuration shown in FIG. 20, for example, the adaptive filteroperation unit 408 a can output the discretization signal where theexternal noise is removed (or the external noise is not included), basedon the discretization signal y3(t) that is output from the BPFindividual unit 406 a.

The synthesizing unit 410 synthesizes the discretization signals outputfrom the adaptive filter operation units 408 a to 408 g, based on thediscretization signals output from the individual BPFs. In addition, thesynthesizing unit 410 can output the discretization signal x′(t) wherethe external noise is removed (or the external noise is not included),based on the discretization signal y(t) that is output from the A/Dconverter 128.

FIGS. 21A and 21B are diagrams illustrating an effect of when acommunication device 400 according to a fourth embodiment of the presentinvention includes a noise removing unit 402. FIG. 21A shows an exampleof a result of a de-spread process in the demodulating unit 108, whenthe communication device according to the embodiment of the presentinvention uses the frequency sampling filter that does not have theadaptive filter operation units 408 a to 408 g, that is, W(f)=1 isapplied. FIG. 21B shows an example of a result of a de-spread process inthe demodulating unit 108, when the communication device according tothe embodiment of the present invention includes the noise removing unit402, that is, the communication device uses the frequency samplingfilter that includes the adaptive filter operation units 408 a to 408 g.FIGS. 21A and 21B show a result of a de-spread process in thedemodulating unit 108 when the A/D converter 128 outputs thediscretization signal y(t) indicated by the output spectrum shown inFIG. 12.

As shown in FIGS. 21A and 21B, when the communication device accordingto the embodiment of the present invention does not include the noiseremoving unit 402, that is, when the communication device uses thefrequency sampling filter that does not have the adaptive filteroperation units 408 a to 408 g, that is, when the frequency samplingfilter is applied as W(f)=1, the spectrum spread signal is not detectedin the demodulating unit 108 (refer to FIG. 21A). Meanwhile, when thecommunication device according to the embodiment of the presentinvention includes the noise removing unit 402, that is, the frequencysampling filter including the adaptive filter operation units 408 a to408 g is used, it can be recognized that the spectrum spread signal isdetected in the demodulating unit 108 (refer to FIG. 21B).

Accordingly, the noise removing unit 402 can function as an adaptivefilter that can remove the broadband noise (external noise) shown inFIG. 12. The communication device 400 according to the fourth embodimentof the present invention does not remove the external noise byperforming a fast Fourier transform (FFT) or an inversed fast Fouriertransform (IFFT), as in the communication devices according to thesecond and third embodiments. However, similar to the communicationdevices according to the second and third embodiments, as shown in FIG.21B, an external noise can be removed. Further, in the communicationdevices according to the second and third embodiments, the example ofthe detection result of the spectrum spread signal in the demodulatingunit 108 is not shown. However, similar to the noise removing unit 402according to the fourth embodiment, the communication devices accordingto the second and third embodiments use the Wiener filter operation toremove the external noise. That is, even in the communication devicesaccording to the second and third embodiments, it is possible to obtainthe same effect as FIG. 21B.

Since the communication device 400 includes, for example, the noiseremoving unit 402 shown in FIG. 20 to remove the external noise, thecommunication device 400 can detect and demodulate the spectrum spreadsignal, as shown in FIG. 21B.

As described above, the communication device 400 according to the fourthembodiment of the present invention basically has the same configurationas the communication device 100 according to the first embodiment shownin FIG. 6, and receives the transmission signal transmitted from theexternal device, detects the spectrum spread signal included in thereceived transmission signal, and demodulates the spectrum spreadsignal. Further, the communication device 400 includes the noiseremoving unit 402 that has a plurality of BPFs whose central frequenciesare different from each other by the amount that can cover an IF signalband and performs a Wiener filter operation on each output of the BPFs.In this case, similar to the noise removing unit 202 according to thesecond embodiment shown in FIG. 13, the noise removing unit 402 uses theWiener filter to remove the external noise with respect to thediscretization signals output from the individual BPFs. That is, sincethe noise removing unit 402 does not need to perform a feedbackoperation on the discretization signal in order to detect the externalnoise, the communication device does not perform an unstable operationas in the communication device 10 according to the related art.Accordingly, the communication device 400 can stably remove the externalnoise from the transmission signal where the spectrum spread signal ismodulated. Further, since the communication device 400 can stably removethe external noise, it is possible to surely demodulate the spectrumspread signal.

Further, similar to the communication device 100 according to the firstembodiment, in the communication device 400, the A/D converter 128 thatis provided at the final stage of the analog circuit to process ananalog signal is composed of an A/D converter that has a resolution of Nbits that are larger than the number of bits corresponding to averageamplitude of a normal thermal noise. In addition, the A/D converter 128sets the average amplitude of the normal thermal noise as lower M bitsof the A/D converter 128. Accordingly, the A/D converter 128 that isincluded in the communication device 400 can prevent the output spectrumof the A/D converter from being saturated due to the external noise, andsurely remove the external noise in the noise removing unit 402 that isprovided at the rear stage of the A/D converter 128.

(Program According to a Fourth Embodiment)

The external noise can be stably removed from the transmission signalwhere the spectrum spread signal is modulated, by using a program thatallows a computer to function as the noise removing unit 402 of thecommunication device 400 according to the fourth embodiment.

(Fifth Embodiment)

In the above case, as the communication device according to the firstembodiment of the present invention, the description has been given tothe communication device including the noise removing unit that mainlyremoves a single frequency noise or a narrowband noise where a noiseband is narrow. Further, in the above case, as the communication devicesaccording to the second to fourth embodiments, the description has beengiven to the communication devices each including the noise removingunit that function as an adaptive filter that can remove a broadbandnoise. However, the configuration of the noise removing unit that isincluded in the communication device according to the embodiment of thepresent invention is not limited to the configuration of the noiseremoving unit according to the first to fourth embodiments. For example,the communication device according to the embodiment of the presentinvention may include a noise removing unit having the configurationwhere the configuration of the noise removing unit according to thefirst embodiment and the configurations of the noise removing unitsaccording to the second to fourth embodiments are combined.

Hereinafter, the configuration of a communication device (hereinafter,referred to as a “communication device 500”) according to a fifthembodiment of the present invention will be described. In this case, thecommunication device 500 and the communication devices according to thefirst to fourth embodiments are different from each other in theconfiguration of the noise removing unit, and the other configurationthereof can be the same as the configurations of the communicationdevices according to the first to fourth embodiments. Accordingly, inthe description below, the configuration of the noise removing unit 502of the communication device 500 is described, and the otherconfiguration is not described.

[Example of the Configuration of a Noise Removing Unit 502]

FIG. 22 is a diagram illustrating an example of the configuration of anoise removing unit 502 according to a fifth embodiment of the presentinvention. In FIG. 22, the communication antenna 102 and the frequencyconverting unit 104 are shown together. In FIG. 22, a discretizationsignal that has the possibility of including an external noise outputfrom the frequency converting unit 104 is shown as a discretizationsignal y(t), and the discretization signal after removing the noise isshown as a discretization signal x(t). In this case, the discretizationsignal x(t) that is output from the noise removing unit 502 istransmitted to the demodulating unit 108.

Referring to FIG. 22, the noise removing unit 502 includes a Fouriertransforming unit 160, a notch frequency detecting unit 182, k (k is aninteger of two or more) notch filters 164 a to 164 k, and an adaptivefilter operation unit 302 (first adaptive filter).

The Fourier transforming unit 160, the notch frequency detecting unit182, and the k (k is an integer of 2 or more) notch filters 164 a to 164k that are included in the noise removing unit have the sameconfiguration as those of the noise removing unit 180 according to themodification of the first embodiment shown in FIG. 10. Accordingly, theFourier transforming unit 160, the notch frequency detecting unit 182,and the k (k is an integer of 2 or more) notch filters 164 a to 164 kcan mainly remove a single frequency noise or a narrowband noise where anoise band is narrow.

Further, the adaptive filter 302 has the same configuration as the noiseremoving unit 302 according to the third embodiment shown in FIG. 15.Accordingly, the adaptive filter 302 can remove a broadband noise.

That is, the noise removing unit 502 is configured by combining thenoise removing unit 180 according to the modification of the firstembodiment and the noise removing unit 302 according to the thirdembodiment. Accordingly, the noise removing unit 502 can remove a singlefrequency noise, a narrowband noise where a noise band is narrow, and abroadband noise.

Further, as described above, each of the noise removing unit 180according to the modification of the first embodiment and the noiseremoving unit 302 according to the third embodiment do not need toperform a feedback operation with respect to the discretization signalsin order to detect the external noise. Accordingly, the noise removingunit 502 that is configured by combining the noise removing unit 180 andthe noise removing unit 302 does not perform an unstable operation evenin the configuration shown in FIG. 22, as in the communication device 10according to the related art.

Accordingly, the communication device 500 includes the noise removingunit 502, thereby stably removing the external noise from thetransmission signal where the spectrum spread signal is modulated.Further, in FIG. 22, the noise removing unit according to the fifthembodiment of the present invention is configured by combining the noiseremoving unit 180 according to the modification of the first embodimentand the noise removing unit 302 according to the third embodiment, butis not limited to the above configuration. For example, the noiseremoving unit according to the fifth embodiment of the present inventioncan be configured by combining the noise removing unit 106 according tothe first embodiment and the noise removing unit 302 according to thethird embodiment, or can be configured by combining the noise removingunit 180 according to the modification of the first embodiment and thenoise removing unit 402 according to the fourth embodiment.

As described above, the communication device 500 according to the fifthembodiment of the present invention basically has the same configurationas the communication device 100 according to the first embodiment shownin FIG. 6, receives the transmission signal transmitted from theexternal device, detects the spectrum spread signal included in thereceived transmission signal, and demodulates the spectrum spreadsignal. Further, the communication device 500 includes the noiseremoving unit 502 that is configured by combining the noise removingunit 180 according to the modification of the first embodiment and thenoise removing unit 302 according to the third embodiment. In this case,since the noise removing unit 502 does not need to perform a feedbackoperation on the discretization signal in order to detect the externalnoise, the communication device 500 does not perform an unstableoperation, as in the communication device 10 according to the relatedart. Accordingly, the communication device 500 can stably remove theexternal noise from the transmission signal where the spectrum spreadsignal is modulated. Further, since the communication device 500 canstably remove the external noise, the communication device 500 cansurely demodulate the spectrum spread signal.

Further, similar to the communication device 100 according to the firstembodiment, in the communication device 500, the A/D converter 128 thatis provided at the final stage of the analog circuit to process ananalog signal is composed of an A/D converter that has a resolution of Nbits that are larger than the number of bits corresponding to averageamplitude of a normal thermal noise. In addition, the A/D converter 128sets the average amplitude of the normal thermal noise as lower M bitsof the A/D converter 128. Accordingly, the A/D converter 128 that isincluded in the communication device 500 can prevent the output spectrumof the A/D converter from being saturated due to the external noise, andsurely remove the external noise in the noise removing unit 502 that isprovided at the rear stage of the A/D converter 128.

[Modification of a Communication Device According to a Fifth Embodiment]

In the above case, as the communication device according to the fifthembodiment of the present invention, the description has been given tothe communication device 500 including the noise removing unit 502 thatcan remove a single frequency noise, a narrowband noise where a noiseband is narrow, and a broadband noise. As described above, the noiseremoving unit 502 shown in FIG. 22 can be configured by combining thenoise removing unit 106 according to the first embodiment and the noiseremoving unit 302 according to the third embodiment, thereby removing asingle frequency noise, a narrowband noise where a noise band is narrow,and a broadband noise. However, the configuration of the noise removingunit that is included in the communication device according to the fifthembodiment of the present invention is not limited to the configurationof the noise removing unit shown in FIG. 22.

As an example of the broadband noise that has the possibility of beingincluded in the discretization signal, an impulsive noise having a shortpersistence time is exemplified. Since the impulsive noise exists on atime base, the impulsive noise can be effectively removed bypressurizing amplitude on the time base. However, even when theamplitude on the time base is pressurized with respect to the impulsivenoise, a noise spreads on a frequency axis, thus, the amplitude may bepressurized over the entire band if the noise passes through an adaptivefilter for a broadband noise (for example, adaptive filter operationunit 302). For this reason, in regards to the impulsive noise, an S/Nratio of the spectrum spread signal that is detected in a de-spreadprocess may be deteriorated due to the adaptive filter.

Accordingly, a communication device (hereinafter, referred to as“communication device 550”) according to a modification of the fifthembodiment of the present invention executes the following processes of(a) and (b) to prevent the S/N ratio of the detected spectrum spreadsignal from being deteriorated (lost).

(a) Detection of an Impulsive Noise

In the impulsive noise, even when amplitude is large, time-averagedpower is reduced. Accordingly, the communication device 550 derives anaverage value or an integration value in a predetermined time length,such as 1 [msec], based on the discretization signal that is output fromthe A/D converter 128. Next, the communication device 550 compares thederived average value or integration value and a reference value used todetermine the impulsive noise. In addition, the communication device 550determines that the impulsive noise is detected, when the derivedaverage value or integration value becomes larger than the referencevalue.

(b) Control of a Filter

When it is determined that the impulsive noise is detected during theprocess of (a), the communication device 550 selectively turns off, forexample, an adaptive filter (for example, adaptive filter 302) for abroadband noise, among the filters that constitute the noise removingunit.

The communication device 550 according to the modification of the fifthembodiment executes the processes of (a) and (b), thereby preventing theS/N ratio from being lost in the adaptive filter for the broadbandnoise, and reducing power consumed in the adaptive filter.

Hereinafter, an example of the configuration of the communication device550 according to the modification of the fifth embodiment will bedescribed. FIG. 23 is a diagram illustrating a portion of an example ofthe configuration of a communication device 550 according to amodification of a fifth embodiment of the present invention. FIG. 23shows a portion of the configuration of the communication device 550. Inthis case, at a rear stage of the noise removing unit 502, for example,a demodulating unit 108 that has the same configuration as thedemodulating unit 108 shown in FIG. 6 is connected.

Referring to FIG. 23, the communication device 550 includes acommunication antenna 102, a frequency converting unit 104, anadjustment signal generating unit 552, and a noise removing unit 502. Inthis case, the communication antenna 102, the frequency converting unit104, and the noise removing unit 502 have the same configurations asthose of the communication device 500 shown in FIG. 22.

The adjustment signal generating unit 552 includes a level detectingunit 554 and a comparing unit 556 (adjustment signal output unit). Thelevel detecting unit 554 derives an average value or an integrationvalue in a predetermined time length, based on a discretization signalthat is output from the A/D converter 128. In this case, the leveldetecting unit 554 may be configured, for example, by using a movementaverage filter, an IIR (Infinite Impulse Response) filter having apredetermined time constant, such as 1 [msec], or an integration circuitby an Integrate & Dump, but the present invention is not limitedthereto.

The comparing unit 556 compares the average value or the integrationvalue in the predetermined time length output from the level detectingunit 554 and the reference value. When the average value or theintegration value is larger than the reference value, the comparing unit556 transmits, for example, an adjustment signal, which selectivelyturns off the adaptive filter 302 of the noise removing unit 502, to thenoise removing unit 502, and selectively turns off the adaptive filter302. In this case, the comparing unit 556 can be composed of, forexample, a comparator, but the present invention is not limited thereto.Further, the reference value is transmitted, for example, by the MPU 144of the demodulating unit 108 or the control unit (not shown). Further,the comparing unit 556 transmits, for example, an adjustment signal thatis generated with respect to a switching unit to selectively connect thediscretization signal to the adaptive filter 302 or the bypass, therebyselectively turning off the adaptive filter 302, but the presentinvention is not limited thereto.

By the above-described configuration, the adjustment signal generatingunit 552 can execute the processes of (a) and (b).

By the configuration shown in FIG. 23, for example, the communicationdevice 550 according to the modification of the fifth embodiment canprevent the S/N ratio of the detected spectrum spread signal from beingdeteriorated.

Further, since the communication device 550 basically has the sameconfiguration as the communication device 500 shown in FIG. 22, it ispossible to achieve the same effect as the communication device 500according to the fifth embodiment.

(Program According to a Fifth Embodiment)

The external noise can be stably removed from the transmission signalwhere the spectrum spread signal is modulated, by using a program thatallows a computer to function as the noise removing unit 502 of thecommunication device 500 according to the fifth embodiment.

(Sixth Embodiment)

In the above case, as the communication devices according to the firstto fifth embodiments of the present invention, the configuration of thecommunication device has been described, in which the A/D converter isincluded at the final stage of the frequency converting unit and has aresolution of N bits that are larger than the number of bitscorresponding to average amplitude of a normal thermal noise, and theaverage amplitude of the normal thermal noise is set as lower M bits. Bythe above configuration, the communication device according to theembodiment of the present invention can prevent the output spectrum ofthe A/D converter from being saturated due to the external noise, andsurely remove the external noise in the noise removing unit that isprovided at the rear stage of the A/D converter.

Further, the communication device according to the embodiment of thepresent invention allows the demodulating unit to execute a de-spreadprocess on the discretization signal where the external noise is removedin the noise removing unit, thereby demodulating the spectrum spreadsignal. In this case, a general GPS receiving device that communicateswith a GPS satellite is composed of a one-bit or two-bit A/D converter.Therefore, for example, in the case of an ideal state where the externalnoise does not exist, or in the case where the level of the externalnoise is sufficiently lower, if the communication device includes theone-bit or two-bit A/D converter, the communication device candemodulate a GPS signal. That is, since the communication deviceaccording to the embodiment of the present invention allows the noiseremoving unit to remove the external noise, the M-bit (for example, 3bits or more) discretization signal is not necessarily needed, in thedemodulating unit that is provided at the rear stage of the noiseremoving unit.

Accordingly, the communication device according to the sixth embodimentof the present invention reduces the number of bits with respect to thediscretization signal where the external noise is removed in the noiseremoving unit. By reducing the number of bits with respect to thediscretization signal where the external noise is removed, thecommunication device according to the sixth embodiment of the presentinvention can reduce the sizes of an operator or a register and a memorythat constitute the demodulating unit. Further, in the case where afunction of reducing the number of bits is added to the communicationdevice 550 according to the modification of the fifth embodiment shownin FIG. 23, even when the communication device according to theembodiment of the present invention selectively turns off, for example,the adaptive filter 302 by the bypass, the communication device canreduce the number of bits to suppress amplitude of an impulsive noisethat is included in the discretization signal. Accordingly, even in theabove case, the communication device according to the embodiment of thepresent invention can surely demodulate the spectrum spread signal.

Hereinafter, an example of the configuration of the communication deviceaccording to the sixth embodiment of the present invention will bedescribed. FIG. 24 is a diagram illustrating a portion of an example ofthe configuration of a communication device (hereinafter, referred to as“communication device 600”) according to a sixth embodiment of thepresent invention. FIG. 24 shows a portion of the configuration of thecommunication device 600. In this case, the demodulating unit 108 thathas the same configuration as the demodulating unit 108 shown in FIG. 6is connected, for example, at a rear stage of the noise removing unit502.

Referring to FIG. 24, the communication device 600 includes acommunication antenna 102, a frequency converting unit 104, a noiseremoving unit 502, and a bit reducer 602 (bit number setting unit). Inthis case, the communication antenna 102, the frequency converting unit104, and the noise removing unit 502 have the same configurations asthose of the communication device 500 shown in FIG. 22. FIG. 24 shows anexample of when a 6-bit discretization signal is output from the A/Dconverter 128 of the frequency converting unit 104.

The bit reducer 602 outputs, for example, a signal where an upper limitis restricted with respect as lower P bits (P is an integer where N>P)of the discretization signal, based on the discretization signal outputfrom the noise removing unit 502, thereby reducing the number of bits ofthe discretization signal. FIG. 24 shows an example of when the bitreducer 602 reduces the number of bits of the discretization signal fromthe discretization signal of 6 bits to the discretization signal of 2bits. Further, a method of reducing the number of bits in the bitreducer 602 according to the sixth embodiment of the present inventionis not limited to the above example.

By the configuration shown in FIG. 24, for example, the communicationdevice 600 according to the sixth embodiment can reduce the number ofbits with respect to the discretization signal where the external noiseis removed in the noise removing unit.

Further, since the communication device 600 basically has the sameconfiguration as the communication device 500 shown in FIG. 22, it ispossible to achieve the same effect as the communication device 500according to the fifth embodiment. In FIG. 24, as the communicationdevice according to the sixth embodiment, the communication device 600having the configuration where the bit reducer 602 is added to thecommunication device 500 shown in FIG. 22 is exemplified, but thepresent invention is not limited thereto. For example, the communicationdevice according to the sixth embodiment of the present invention can beconfigured by additionally providing the bit reducer 602 at the rearstage of the noise removing unit, with respect to the communicationdevices according to the first to fourth embodiments.

(Seventh Embodiment)

Since the notch filter or the Wiener filter that the communicationdevice according to the first to sixth embodiments includes as the noiseremoving unit is a digital filter, in the noise removing units accordingto the first to sixth embodiments, a large ratio between a samplingfrequency of filter processing and a carrier frequency is effective interms of processing. Further, when an intermediate frequency of an IFsignal is not zero, the number of A/D converts that are included in thefrequency converting unit is one. Thus, an intermediate frequency (IF)is generally set as several MHz in the GPS receiving device or the like.

Accordingly, the communication device according to the seventhembodiment of the present invention allows the frequency converting unitto convert a transmission signal into an IF signal having apredetermined frequency other than zero. Next, the communication deviceaccording to the seventh embodiment further converts the discretizationsignal, which is output from the A/D converter and has a predeterminedfrequency, into the discretization signal having a zero frequency. Inaddition, the communication device according to the seventh embodimentremoves an external noise from the discretization signal where afrequency is zero, and demodulates a spectrum spread signal.

Accordingly, the communication device according to the seventhembodiment of the present invention can configure the frequencyconverting unit to include one A/D converter. Further, it is possible toimprove easiness of a process of removing an external noise in the noiseremoving unit. Hereinafter, an example of the configuration of thecommunication device according to the seventh embodiment will bedescribed.

FIG. 25 is a diagram illustrating a portion of an example of theconfiguration of a communication device 700 according to a seventhembodiment of the present invention. FIG. 25 shows a portion of theconfiguration of the communication device 550. In this case, at a rearstage of the bit reducer 602 a and the bit reducer 602 b, for example,the demodulating unit 108 that has the same configuration as thedemodulating unit 108 shown in FIG. 6 is connected.

Referring to FIG. 25, the communication device 700 includes acommunication antenna 102, a frequency converting unit 104, anadjustment signal generating unit 552, a second frequency convertingunit 702 (frequency converting unit), a low-pass filter 704 a, alow-pass filter 704 b, a noise removing unit 502, a bit reducer 602 a,and a bit reducer 602 b. In this case, the communication antenna 102,the frequency converting unit 104, the adjustment signal generating unit552, and the noise removing unit 502 have the same configurations asthose of the communication device 550 shown in FIG. 23. Further, the bitreducer 602 a and the bit reducer 602 b have the same configurations asthose of the communication device 600 shown in FIG. 24. Further, it isassumed that the intermediate frequency converted by the frequencyconverting unit 104 is, for example, a frequency other than zero, suchas 4.092 MHz or 1.023 MHz.

The second frequency converting unit 702 includes an NCO (NumericControlled Oscillator) 710, a multiplier 712 a, and a multiplier 712 b.

The multiplier 712 a multiplies the discretization signal output fromthe A/D converter 128 and a sine component of an oscillation signaltransmitted from the NCO 710. Further, the multiplier 712 b multipliesthe discretization signal output from the A/D converter 128 and thecosine component of the oscillation signal transmitted from the NCO 710.In this case, the NCO 710 generates an oscillation signal whosefrequency corresponds to an intermediate frequency. For example, whenthe frequency (that is, intermediate frequency) of the discretizationsignal that is output from the A/D converter 128 is 4.092 MHz, the NCO710 generates an oscillation signal of 4.092 MHz.

By the above configuration, the second frequency converting unit 702 canconvert the discretization signal of the frequency other than zero intoa discretization signal whose frequency is zero. Further, theconfiguration of the second frequency converting unit 702 is not limitedto the above example.

The low-pass filter 704 a attenuates a signal having a frequency that ishigher than a cutoff frequency, based on the discretization signaloutput from the multiplier 712 a, and in the same way, the low-passfilter 704 b attenuates a signal having a frequency that is higher thana cutoff frequency, based on the discretization signal output from themultiplier 712 b. In addition, the low-pass filter 704 a and thelow-pass filter 704 b output the discretization signals output from themultiplier 712 a and the multiplier 712 b to the noise removing unit502.

The noise removing unit 502 processes the discretization signal where afrequency is converted into zero in the second frequency converting unit702.

By the configuration shown in FIG. 25, for example, the communicationdevice 700 can improve easiness of a process of removing the externalnoise in the noise removing unit. Further, since the communicationdevice 700 has the same configuration as the communication device 550shown in FIG. 23, it is possible to achieve the same effect as thecommunication device 550 according to the modification of the fifthembodiment.

The communication devices 100 to 700 according to the embodiments of thepresent invention have been described, but the embodiments of thepresent invention are not limited thereto. For example, the presentinvention can be applied to a computer, such as an UMPC (Ultra MobilePersonal Computer), a portable communication device, such as a mobilephone, a portable game machine, such as a PlayStation Portable(registered trademark), a navigation device, such as car navigation, andan imaging device, such as a digital still camera.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A noise removing method that can use acommunication device comprising a communication antenna that receives atransmission signal where a spectrum spread signal subjected to aspectrum spread is modulated, an intermediate frequency converting unitthat converts the transmission signal received by the communicationantenna into an intermediate frequency signal having a predeterminedfrequency, and an analog to digital converting unit that discretizes theintermediate frequency signal and outputs a discretization signal, andremoves a noise other than a normal thermal noise included in thediscretization signal, the noise removing method comprising the stepsof: performing a fast Fourier transform on the discretization signal;detecting a frequency whose amplitude is a predetermined value or moreas a peak frequency of the noise, based on a result of the fast Fouriertransform in the performing of the fast Fourier transform; andoutputting a discretization signal where a frequency componentcorresponding to the peak frequency detected in the detecting of thefrequency is attenuated, based on the discretization signal, and withoutperforming a feedback operation on the outputted, discretization signal.2. The noise removing method of claim 1, further comprising: outputtinga discretization signal that minimizes a mean squared error with respectto an ideal discretization signal in an ideal state not including thenoise; detecting an average value or an integration value in apredetermined time of the discretization signal; and outputting anadjustment signal to selectively control outputting of thediscretization signal based on the average value or the integrationvalue and a predetermined threshold value.
 3. The noise removing methodof claim 1, wherein detecting the frequency comprises detecting aplurality of frequencies, and the method further comprises setting thedetected peak frequencies in the order of the peak frequencies havinglarge amplitude.
 4. The noise removing method of claim 1, furthercomprising: selectively outputting a result of the fast Fouriertransform or a power spectrum derived from the result of the fastFourier transform based on the result of the fast Fourier transform andthe power spectrum; when the power spectrum is output, outputting adiscretization signal that minimizes a mean squared error with respectto an ideal discretization signal in an ideal state not including thenoise based on the power spectrum and reference power per unitfrequency; and performing an inversed fast Fourier transform on theresult of the fast Fourier transform or the discretization signal. 5.The noise removing method of claim 1, further comprising detecting aplurality of predetermined frequency band detection signals from thediscretization signal; for each frequency band detection signal,selectively outputting the detection signal or a discretization signalthat minimizes a mean squared error with respect to an idealdiscretization signal in an ideal state not including the noise; andsynthesizing the discretization signals, wherein selectively outputtingthe detection signal or the discretization signal comprises: selectivelyoutputting the detection signal or a power spectrum derived from thedetection signal based on the power spectrum; and outputting adiscretization signal that minimizes a mean squared error with respectto the ideal discretization signal based on the power spectrum andreference power per unit frequency, when the power spectrum is output.6. The noise removing method of claim 1, further comprising: convertingan input analog signal into a digital signal with a resolution of N bits(N is an integer) larger than the number of bits corresponding toaverage amplitude of the normal thermal noise; and setting the averageamplitude of the normal thermal noise as lower M bits (M is an integer,where N>·M).
 7. The noise removing method of claim 6, furthercomprising: setting the number of bits of the discretization signal as Pbits (P is an integer where N>·P).
 8. A noise removing method that canuse a communication device comprising a communication antenna thatreceives a transmission signal where a spectrum spread signal subjectedto a spectrum spread is modulated, an intermediate frequency convertingunit that converts the transmission signal received by the communicationantenna into an intermediate frequency signal having a predeterminedfrequency, and an analog to digital converting unit that discretizes theintermediate frequency signal and outputs a discretization signal, andremoves a noise other than a normal thermal noise included in thediscretization signal, the noise removing method comprising the stepsof: performing a fast Fourier transform on the discretization signal andderiving a power spectrum based on a result of the fast Fouriertransform; and outputting a discretization signal that minimizes a meansquared error with respect to an ideal discretization signal in an idealstate not including the noise, based on the discretization signal, thepower spectrum derived from the fast Fourier transform, reference powerper unit frequency, and without performing a feedback operation on theoutputted discretization signal.
 9. The noise removing method of claim8, further comprising: detecting a frequency whose amplitude is apredetermined value or more as a peak frequency of the noise, based on aresult of the fast Fourier transform; and outputting a discretizationsignal where a frequency component corresponding to the peak frequencyis attenuated, based on the discretization signal.
 10. The noiseremoving method of claim 9, further comprising: detecting an averagevalue or an integration value in a predetermined time of thediscretization signal; and outputting an adjustment signal toselectively control outputting of the discretization signal based on theaverage value or the integration value and a predetermined thresholdvalue.
 11. The noise removing method of claim 9, wherein detecting thefrequency comprises detecting a plurality of frequencies, and the methodfurther comprises setting the detected peak frequencies in the order ofthe peak frequencies having large amplitude.
 12. The noise removingmethod of claim 8, further comprising: selectively outputting a resultof the fast Fourier transform or a power spectrum derived from theresult of the fast Fourier transform based on the result of the fastFourier transform and the power spectrum; outputting the discretizationsignal based on the power spectrum and reference power per unitfrequency, when the power spectrum is output; and performing an inversedfast Fourier transform on the result of the fast Fourier transform orthe discretization signal.
 13. The noise removing method of claim 8,further comprising detecting a plurality of predetermined frequency banddetection signals from the discretization signal; for each frequencyband detection signal, selectively outputting the detection signal orthe discretization signal; and synthesizing the discretization signals,wherein selectively outputting the detection signal or thediscretization signal comprises: selectively outputting the detectionsignal or a power spectrum derived from the detection signal based onthe power spectrum; and outputting the discretization signal based onthe power spectrum and reference power per unit frequency, when thepower spectrum is output.
 14. The noise removing method of claim 8,further comprising: converting an input analog signal into a digitalsignal with a resolution of N bits (N is an integer) larger than thenumber of bits corresponding to average amplitude of the normal thermalnoise; and setting the average amplitude of the normal thermal noise aslower M bits (M is an integer, where N>·M).
 15. The noise removingmethod of claim 14, further comprising: setting the number of bits ofthe discretization signal as P bits (P is an integer where N>·P).
 16. Anon-transitory computer readable storage device storing a computerprogram for use with a communication device comprising a communicationantenna that receives a transmission signal where a spectrum spreadsignal subjected to a spectrum spread is modulated, an intermediatefrequency converting unit that converts the transmission signal receivedby the communication antenna into an intermediate frequency signalhaving a predetermined frequency, and an analog to digital convertingunit that discretizes the intermediate frequency signal and outputs adiscretization signal, and removes a noise other than a normal thermalnoise included in the discretization signal, the program which whenexecuted by a computer, performs a noise removing method, the methodcomprising: performing a fast Fourier transform on the discretizationsignal; detecting a frequency whose amplitude is a predetermined valueor more as a peak frequency of the noise, based on a result of the fastFourier transform in the performing of the fast Fourier transform; andoutputting a discretization signal where a frequency componentcorresponding to the peak frequency detected in the detecting of thefrequency is attenuated, based on the discretization signal, and withoutperforming a feedback operation on the outputted discretization signal.17. The program of claim 16, further allowing the computer to execute:outputting a discretization signal that minimizes a mean squared errorwith respect to an ideal discretization signal in an ideal state notincluding the noise; detecting an average value or an integration valuein a predetermined time of the discretization signal; and outputting anadjustment signal to selectively control outputting of thediscretization signal based on the average value or the integrationvalue and a predetermined threshold value.
 18. The program of claim 16,wherein detecting the frequency comprises detecting a plurality offrequencies, and the method further comprises setting the detected peakfrequencies in the order of the peak frequencies having large amplitude.19. The program of claim 16, further allowing the computer to execute:selectively outputting a result of the fast Fourier transform or a powerspectrum derived from the result of the fast Fourier transform based onthe result of the fast Fourier transform and the power spectrum; whenthe power spectrum is output, outputting a discretization signal thatminimizes a mean squared error with respect to an ideal discretizationsignal in an ideal state not including the noise based on the powerspectrum and reference power per unit frequency; and performing aninversed fast Fourier transform on the result of the fast Fouriertransform or the discretization signal.
 20. The program of claim 16,further comprising detecting a plurality of predetermined frequency banddetection signals from the discretization signal; for each frequencyband detection signal, selectively outputting the detection signal or adiscretization signal that minimizes a mean squared error with respectto an ideal discretization signal in an ideal state not including thenoise; and synthesizing the discretization signals, wherein selectivelyoutputting the detection signal or the discretization signal comprises:selectively outputting the detection signal or a power spectrum derivedfrom the detection signal based on the power spectrum; and outputting adiscretization signal that minimizes a mean squared error with respectto the ideal discretization signal based on the power spectrum andreference power per unit frequency, when the power spectrum is output.21. The program of claim 16, further allowing the computer to execute:converting an input analog signal into a digital signal with aresolution of N bits (N is an integer) larger than the number of bitscorresponding to average amplitude of the normal thermal noise; andsetting the average amplitude of the normal thermal noise as lover Mbits (M is an integer, where N>·M).
 22. The program of claim 21, furtherallowing the computer to execute: setting the number of bits of thediscretization signal as P bits (P is an integer where N>·P).
 23. Anon-transitory computer readable storage device storing a computerprogram for use with a communication device comprising a communicationantenna that receives a transmission signal where a spectrum spreadsignal subjected to a spectrum spread is modulated, an intermediatefrequency converting unit that converts the transmission signal receivedby the communication antenna into an intermediate frequency signalhaving a predetermined frequency, and an analog to digital convertingunit that discretizes the intermediate frequency signal and outputs adiscretization signal, and removes a noise other than a normal thermalnoise included in the discretization signal, the program which whenexecuted by a computer, performs a noise removing method, the methodcomprising: performing a fast Fourier transform on the discretizationsignal and deriving a power spectrum based on a result of the fastFourier transform; and outputting a discretization signal that minimizesa mean squared error with respect to an ideal discretization signal inan ideal state not including the noise, based on the discretizationsignal, the power spectrum derived from the fast Fourier transform,reference power per unit frequency, and without performing a feedbackoperation on the outputted discretization signal.
 24. The program ofclaim 23, further allowing the computer to execute: detecting afrequency whose amplitude is a predetermined value or more as a peakfrequency of the noise, based on a result of the fast Fourier transform;and outputting a discretization signal where a frequency componentcorresponding to the peak frequency is attenuated, based on thediscretization signal.
 25. The program of claim 24, further allowing thecomputer to execute: detecting an average value or an integration valuein a predetermined time of the discretization signal; and outputting anadjustment signal to selectively control outputting of thediscretization signal based on the average value or the integrationvalue and a predetermined threshold value.
 26. The program of claim 24,wherein detecting the frequency comprises detecting a plurality offrequencies, and the method further comprises setting the detected peakfrequencies in the order of the peak frequencies having large amplitude.27. The program of claim 23, further allowing the computer to execute:selectively outputting a result of the fast Fourier transform or a powerspectrum derived from the result of the fast Fourier transform based onthe result of the fast Fourier transform and the power spectrum;outputting the discretization signal based on the power spectrum andreference power per unit frequency, when the power spectrum is output;and performing an inversed fast Fourier transform on the result of thefast Fourier transform or the discretization signal.
 28. The program ofclaim 23, further comprising detecting a plurality of predeterminedfrequency band detection signals from the discretization signal; foreach frequency band detection signal, selectively outputting thedetection signal or the discretization signal; and synthesizing thediscretization signals, wherein selectively outputting the detectionsignal or the discretization signal comprises: selectively outputtingthe detection signal or a power spectrum derived from the detectionsignal based on the power spectrum; and outputting the discretizationsignal based on the power spectrum and reference power per unitfrequency, when the power spectrum is output.
 29. The program of claim23, further allowing the computer to execute: converting an input analogsignal into a digital signal with a resolution of N bits (N is aninteger) larger than the number of bits corresponding to averageamplitude of the normal thermal noise; and setting the average amplitudeof the normal thermal noise as lower M bits (M is an integer, whereN>·M).
 30. The program of claim 29, further allowing the computer toexecute: setting the number of bits of the diseretization signal as Phits (P is an integer where N>·P).